CHICAGO — 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% of women, the researchers who spy on people in public restrooms said. They reported their latest findings today 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% of women.

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

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% of Americans said they usually or always wash up after using the bathroom.

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

The dirty details:

— Atlanta’s Turner Field baseball stadium again was the worst. Only 57% of guys there washed up, compared to 95% 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% of men and women combined washed their hands, compared to 79% at the city’s Penn and Grand Central train stations.

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

Carry 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. The latest was of 1,001 adults between August 17 and 20.

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