Urine samples were collected from 27 dogs and 4 cats with indwelling urinary catheters connected to a closed system. Twenty-one animals had sterile urine at the initiation of catheterization. Eleven of these 21 animals developed positive urine culture after 4 (mean) days (range 1 to 10 days) of catheterization. Infections persisted after catheter removal in 6 of 8 animals. The 10 animals that did not develop bacteriuria had a shorter duration of catheterization (3 days vs 12 days, mean). Antibiotics (chloramphenicol, trimethoprim/sulfonamide, cephradine, lincomycin, gentamicin, or ampicillin) were used during the period of catheterization in 17 of 21 animals, 8 of which developed bacteriuria and 9 of which did not. Ten animals had bacteriuria at the initiation of urine collection for culture. The type of bacteria changed during the catheterization period in 6 animals, all of which were given antibiotics. Bacteria isolated from animals given antibiotics seemed to become increasingly antibiotic-resistant.