CHICAGO &#8212 The gender gap has widened when it comes to hygiene, according to the latest stakeout by the “hand-washing police.”

One-third of men didn’t bother to wash after using the bathroom, compared with 12 percent of women, said the researchers who spy on people in public restrooms. They reported their latest findings Monday at a meeting of infectious disease scientists.

Two years ago, the last time the survey was done, only one quarter of men didn’t wash, compared with 10 percent of women.

“Guys need to step up to the sink,” said Brian Sansoni, spokesman for the Soap and Detergent Association, which co-sponsors the survey and related education campaigns.

The latest study was based on observations last month of more than 6,000 people in four big cities.

Frequent hand washing is the single best thing people can do to avoid getting sick, from colds and the flu to germs lurking in food, doctors say. And a recent Harris Interactive survey found 92 percent of Americans said they always wash up after using the bathroom.

But researchers for the American Society for Microbiology found that only 77 percent actually do, when it comes to public restrooms. That’s a 6 percent decline from a similar study in 2005.

The dirty details:

– Atlanta’s Turner Field baseball stadium again was the worst. Only 57 percent of guys there washed up, compared to 95 percent of women.

– New York was Second City to Chicago in cleanliness. In restrooms at the Windy City’s Shedd Aquarium and Museum of Science and Industry, 81 percent of men and women combined washed their hands, compared to 79 percent at the Big Apple’s Penn and Grand Central train stations.

– At San Francisco’s Ferry Terminal Farmers Market, 62.5 percent of men lathered up. Women did better, with 84 percent.

Carrying sanitizer gels and wipes in case the means to wash your hands aren’t handy, suggested microbiologist Judy Daly of Primary Children’s Medical Center in Salt Lake City, who led the project.

“These are a marvelous addition to plain soap and water,” she said.

Telephone surveys by the research firm Harris Interactive show little shift in attitudes over previous polls in 2003 and 2005.

Nearly three fourths of Americans said they always wash up after changing a diaper, 78 percent said they do so before handling or eating food; 42 percent after petting a dog or cat, 25 percent after handling money, and 34 percent after coughing or sneezing.