The cockroach problem is much worse than you thought.

Apartments in a recent survey were found to have 20,000 cockroaches -- each.

That old bit of kitchen wisdom that for every roach you see, there are 10 you don't see -- forget it. For every one you see there can be 1,000 you don't see, according to Richard S. Patterson, who studies cockroaches in the Agriculture Department's Insects Affecting Man and Animals Laboratory.

The recent survey was done in 550 low-income apartments in Gainesville, Fla., but Patterson added that it is not only low-income housing that has high populations of roaches. Work in the District and elsewhere shows that houses or apartments of the wealthy are also counted as potential roach havens because much food is stored and the kitchens are often run like restaurants.

Patterson said the IAMAL laboratory's test kitchens have repeatedly released thousands of roaches and then monitored how many stray into roach traps best known under the trade name of one, the Roach Motel.

After many trials, the lab calculated the ratio: for each one caught in a trap set out for 24 hours, another 800 roaches were living in the kitchen.

In the apartment survey, the sampling was done by putting five roach traps in each of 550 apartments. In the worst cases, he said, as many as 600 roaches were found flooding a single trap. Even Patterson was stunned.

He said he keeps roach traps in his own house as monitors. "If you find more than one or two, you've got serious problems," he said.