The National Zoo has launched a new attack on the rats and mice that have long menaced the animal park, stepping up efforts to trap and poison the rodents and plugging holes in buildings where they have had easy access to many indoor exhibits.

The assaults on rodents have included placing snap traps near zoo dumpsters and animal areas that attract rats, setting out more than 80 bait boxes around the park and flooding some rat holes.

Feeding stations have been elevated above exhibit floors, where feasible. Wire-mesh ceilings on some enclosures have been reinforced with coverings with holes small enough to keep mice out. Gaps under entryways have been filled with door strips, and many birds now eat from bowls placed on stands in their ponds.

"It's a work in progress to decrease the rodent population, not an overnight change," said Suzy Alberts, the zoo's pest management specialist. "We still have pockets [of infestation], but it's much better."

Some rodents die in traps, while others eat poisoned bait and die elsewhere, so Alberts can't estimate the number of rodents killed. Citing one example, the zoo said that in January it trapped 26 mice in one night in the saki monkey and tamandua anteater exhibit, which is now free of rodents.

Like other animal parks, the 163-acre National Zoo, which is part of the Smithsonian Institution, constantly battles rodents and insects. But its marauding rats have killed prairie dogs and traipsed brazenly through animal yards. Visitors to the zoo last spring and fall reported watching mice crawl over sleeping gorillas to grab the apes' food -- a sight that one patron said "gave me the heebie-jeebies."

In November, a colobus monkey died from leptospirosis, a bacterial disease transmitted by rats and other wild animals.

The rodent problem has drawn complaints from the U.S. Department of Agriculture and the National Academy of Sciences, which has been conducting a study of animal care at the zoo. In an interim report in February, the science panel warned that pest control was inadequate and "poses a potential threat to the animal collection, employees and visitors."

The latest rodent-control efforts come a year and a half after the accidental poisoning of two male red pandas, an endangered species. The animals died in January 2003 after a pesticide contractor buried poison gas pellets in rat holes in their yard in an aggressive attempt to fumigate the rats.

A zoo pathologist found that the pandas had eaten tiny pieces of the poison that fell on the ground while the pellets were being buried.

After the red panda deaths, zoo officials fired the contractor and abruptly halted the use of rodent poison anywhere in the park, including non-animal areas -- a decision that employees say allowed the grounds to be overrun by vermin.

"Not having a contractor here for so long, everything had gotten out of control, from the rats down to the flies and mosquitoes," said Alberts, an entomologist.

Zoo Director Lucy H. Spelman hired Alberts as part of an overhaul of pest control operations she ordered after the red pandas died.

Alberts said that when she started her job in November, she found so many problems that she didn't know where to start. Walking the zoo grounds recently, she pointed out numerous exhibits that had been besieged by rodents.

"This display was just loaded with mice -- they were breeding in the holes of the logs," Alberts said, stopping in front of the golden lion tamarins at the Small Mammal House.

Mice, she said, "were coming through the front door" at the Small Mammal and Amazonia exhibits, squeezing under narrow gaps at the door's base. She showed how rats had cut a path in the grass to the Ape House and how some had climbed into the building through the orangutan yard.

Rats also were in the flight cage at the Bird House, and mice were living in the fiberglass insulation around water pipes on Lemur Island, she said.

Alberts has consulted federal pesticide specialists and sought advice from several other zoos. She brought in a wildlife expert from Fairfax County and joined him one night as he surveyed the park with an infrared camera to see where rats were hiding. This year, Alberts and Prince Seabron, the zoo's pest control officer, as well as keepers, curators and the maintenance crew, began an intensive program for reducing rodents.

The initial emphasis was on mechanical traps that snap shut with lethal force. More recently, the zoo started paying $5,000 a month to another private contractor, Steritech, a firm that has done other work for the Smithsonian, to oversee the use of poison bait boxes and other pest control efforts.

"You try the least toxic way first, mechanical removal of the rodents," Alberts said. "Then, you go to rodenticides."

A committee of zoo employees, including veterinarians and Spelman, now must sign off on all chemicals used in the park. Most of the traps and bait boxes are put out at night or before dawn and removed each day before the zoo opens. No rat poison is permitted in any animal yard or exhibit.

Amazonia will undergo more rodent-proofing in August when the exhibit is shut down for renovations. But much of the rodent work in the Elephant House is on hold because animals there will be moved in a year as part of the future Asia Trail opening.

Still to come are efforts to target rodents in sewers and the basements of buildings.

While rodents are the top priority, Alberts is also working to reduce insects. A pesticide expert from the University of Maryland comes twice a week to spray for yellow jackets, a particular concern for visitors. Kandula, the zoo's young male elephant, is extremely sensitive to stings.

At the cheetah exhibit, stable flies are being killed by a device that uses certain wavelengths of light to attract them to a "sticky board," Alberts said.

The zoo recently acquired two new female red pandas, now in quarantine. One will go on exhibit July 12. The other, however, is being treated for severe dental problems and will not be exhibited, the zoo said. The zoo hopes to acquire a healthier red panda soon.

Alberts said the revamped red panda enclosure will have a black rat snake to discourage rodents, a tactic that has worked well in the zoo's bear exhibit.

The National Zoo uses rodent traps, like these next to a dumpster, as one way to stop vermin. The zoo is also setting poisoned bait, flushing dens and plugging holes.The National Zoo's pest management specialist, Suzy Alberts, discovers an area at the zoo where rats have created a path in the grass.