The significance of coccidiosis in the cat is discussed and the clinical signs of infection are described. The occurrence of subclinical infection with Isospora felis in 49 of 58 cats in a colony is reported. Treatment with a 20% solution of sulphadimethoxine at a dose rate of 50 mg per kg bodyweight, administered in the food daily for 14 days, was successful in eradicating the infection. Sulphadimidine appeared to have no effect on the number of oocysts in the faeces of one cat. Ancillary hygienic measures included the use of disposable feeding and drinking utensils, frequent removal and destruction of faeces and scrubbing of sanitary trays and cages with hot 5% sodium carbonate solution.