In a clinical trial involving 123 cats and 88 dogs, the efficacy of tablets containing nitenpyram against natural flea infestations was investigated. The animals were selected from the routine cases of nine veterinary clinics in the UK and 143 were treated with the tablets and 68 control animals were treated with placebo tablets. Each animal was maintained in an individual cage. The time when the first fleas fell off each animal was recorded between 30 minutes and five hours after treatment, and six hours after treatment the numbers of live, moribund or dead fleas on each animal were determined, and the flea survival rate was calculated. The drug's efficacy was assessed by comparing the mean survival rates of fleas on the treated and control animals. Fleas started to fall from the animals 30 minutes after treatment and two hours after treatment some fleas had detached from 81 per cent of the treated animals. After six hours the efficacy of the drug reached 96.7 per cent on dogs and 95.2 per cent on cats, and 85.9 per cent of the fleas were found off the treated animals, compared with 1.8 per cent in the controls. No adverse drug reactions were recorded during the trial.