The bacterial flora from the jejunum and colon of 19 dogs with naturally occurring exocrine pancreatic insufficiency (EPI) was examined. In six dogs with EPI that did not receive any treatment the bacterial count in the small intestine was > 10(6) colony-forming units (CFU)/g. This was consistent with bacterial overgrowth (BOG) (control dogs: < 10(5) CFU/g). Six dogs with EPI that received pancreatic enzyme supplementation and were in good condition had bacterial counts (> 10(6) CFU/g) that were nonsignificantly different from the untreated group. The remaining seven dogs with EPI that received both pancreatic enzyme supplementation and oral antibiotic therapy (tylosin), had bacterial flora counts that were significantly lower than the values in the other two groups of EPI dogs, and did not differ significantly from those of the control group. There were no statistical differences between the number of bacteria in the colon among the different groups of dogs with EPI and the control dogs. We conclude that the prevalence of BOG in dogs with EPI is high. Pancreatic enzyme supplementation did not affect the microflora of dogs with naturally occurring EPI. Oral antibiotic therapy with tylosin, however, effectively decreased BOG in these dogs.