BERKELEY - Even if students haven't lived in UC Berkeley co-ops, they have stories to tell.

Junior Vincente Kukua once heard about "people moving in at the beginning of the year and finding jars of urine on the counter -- and rats."

Vanessa Wiseman, also a junior: "I've heard horror stories about bums living there, just crashing there."

And junior Pamela Bachilla: "Some have the reputation of being dirty and some party co-ops and some mellow. Those are reputations more than anything. My friends who live in co-ops aren't dirty or crazy."

Students can live in dorms. They can live in apartments. Or they can live in the cheapest option of all, co-ops. In cooperative housing, students buy food, cook meals and clean their common spaces together. Cal's co-ops, founded in 1933 and now 20 in number, are the biggest university co-op system in the United States. They range from cavernous buildings to small former mansions to apartment buildings; they house 4.5 percent of the undergraduate and graduate population.

Often, people choose to live in co-ops for economic reasons but end up finding a community that's different from anything they could find living in the dorms or an apartment on their own.

At Casa Zimbabwe, or CZ, 124 men and women take care of an enormous building on Ridge Road with walls covered in murals and a common room stuffed with tired-looking couches. As at other co-ops, residents each work five hours a week in the building, whether cleaning the bathrooms, mopping the floors or cooking a massive dinner.

"People really get to know each other here," said junior Austin Shapiro. "There's a sense we're all operating as a unit."

At a recent CZ weekly council meeting, everyone gathered to discuss issues ranging from recycling to meals. CZ's "waste reduction manager" rooted through a garbage bag and pulled out recyclables to excite people's consciences. Someone asked permission to host a party in the basement Saturday night. Another resident requested the house spend $40 on the newspaper "Revolutionary Worker."

Shapiro, the house secretary, replied "I could get someone to come talk to us and deliver straight communist propaganda for free." Someone else said they could just get propaganda from the New York Times. Disagreement broke out over that statement.

Another person suggested the house spend the money on three-foot-high balls, which CZ once did until the balls bounced around so much they started to wear out and eventually the duct tape covering the holes stopped working. After much back and forth, someone called for a vote. The motion failed.

"If people aren't tuned into democracy vibes or hippie vibes or whatever we've got here, they're still tuned into how they're own money is spent," Shapiro said. "People care a lot about that."

Rent for the co-ops is roughly $625 a month, including food. That's less than 50 percent of what a bed in the dorms costs. One reason it's so cheap is that the University Students' Cooperative Association is a nonprofit and doesn't pay income or property taxes. Moreover, it has owned a number of its buildings for a very long time.

More than 330 people are on the co-op waiting list for fall semester, but during the severe housing crunch of a couple of years ago, the list topped 1,000.

"There are people who couldn't go to college unless they lived in a co-op," said housing supervisor Betsy Putnam at the central office, where the management team includes a number of former co-opers, some who have worked at the association for more than 20 years.

Residents at the co-ops tend to be socially progressive. They organized a large protest against the war in Iraq. "Green energy" is a big concern; they've had solar hot water heaters on more than half their buildings for two decades. Lothlorien, named for a forest in J.R.R. Tolkien's Lord of the Rings, is the sole vegetarian co-op and allows Food Not Bombs to cook meals in its kitchen weekly for homeless people.

The co-ops range in size from 17 to 151 students, not including apartment buildings. Some are geared toward certain populations. Two are just for women. There are the African-American Theme House and the Oscar Wilde House for gay, lesbian, bisexual and transgender students. Some are known as relatively quiet, such as Hoyt Hall. Others are not. Le Chateau is facing 22 lawsuits from a group of neighbors angry over the noise and concerned about their property values.

One of the biggest, Cloyne Court, has the reputation as a place to party. A student on Lower Sproul Plaza described it this way: "It's kind of dirty, but fun. They have pretty good parties. There's a lot of drinking and drug use."

Two residents of Cloyne barred entry to their co-op, saying no one was allowed to do a story on it. Parents, they said, would read the article and ask, "Is it habitable?"

Having dozens of overworked students live in a space that no one is paid to clean can lead to dirt buildup if people skip their chores. But it's not as bad as the worst rumors suggest. One afternoon at CZ, one member was meticulously scraping the grill clean, and the main kitchen work area had been wiped spotless.

A woman was making a salad for lunch, another resident a turkey burrito and someone else an omelet. Dinner is the only meal cooked communally. The rest of the time, the refrigerators are open to the house and people can make whatever they want.

The community of co-ops is what attracted senior Marisa Brandt, who wears a button on her backpack that reads, "I love my co-op." She had spent her freshman year in the Clark Kerr dorms on campus, which she thought were beautiful but isolating.

"It wasn't the Berkeley that I thought I was coming all this way to find. ... I wasn't seeing the kind of diverse, interested-in-a-lot-of-things, engaged people that I was hoping to find on this campus."

She was placed in Stebbins Hall by the central office, which allows people to rank where they want to live but ultimately decides. The more time someone has spent in the co-op system, the more clout they have in choosing a home. Brandt found what she was looking for.

She plans on writing a book about co-ops. In it, she will delve into the well-known co-op phenomenon of "housecest," or dating someone you live with. It's common, but if things go wrong, it can make for a supremely uncomfortable situation. In Stebbins, someone did a flow chart of 32 people and 22 lines connecting them. Brandt called them "repeat offenders."

"It's been proposed as a mural, but I think it would sort of be horrifying," she said.

There is much that is random about the co-ops. Cloyne is known for having naked Olympics. Lothorien has Naked Pizza Fridays. Stebbins tends to keep its clothes on, but Brandt said the occasional nudist does get a "smile and nod."

CZ last year rented a Green Tortoise bus and took a road trip to Los Angeles so members could sit in the audience of "The Price is Right" wearing homemade T-shirts that said things like "Certainty Makes Madness." One of their own was called to be a contestant and actually won a truck.

"It's a shame people can only live in this environment as students," said Shapiro. "But I can understand why it wouldn't be popular with an older crowd."