The influence of supplemental Bacillus subtilis C-3102 on chronic diarrhea was studied in 40 privately-owned dogs subjected to a double-blind, placebo-controlled trial. After a run-in period of one week, 20 dogs received either placebo or test tablets for four weeks. Test tablets contained 1.3 x 10⁸ colony forming units (CFU) Bacillus subtilis C-3102 per tablet. Both supplements were administered at a rate of half a tablet per 100 g of air-dry habitual diet, which equals 100 ppm Calsporin in the diet. The owners scored fecal characteristics on a line from 0 to 10 cm. For each dog and each variable, the change over time versus baseline was calculated. In comparison with the control treatment, administration of the probiotic did not influence fecal consistency, color and appearance of blood and mucus. The probiotic significantly improved fecal odor (P = 0.037) and tended to reduce flatulence incidence (P = 0.082) and apparent fecal fermentation (P = 0.080). There was no treatment effect on the degree of tenesmus, failed defecation, fecal volume and coprophagy. The probiotic tended to improve coat condition (P = 0.058). Dogs with higher severity degrees of diarrhea during the run-in period showed more improvement during the test period (P = 0.002), this relationship being clearly perceptible for the dogs receiving probiotic treatment (P = 0.056). It is concluded that administration of Bacillus subtilis C-3102 is an effective adjuvant therapy to the treatment of chronic diarrhea in dogs. It is suggested that a well-formulated dog food supplemented with Bacillus subtilis C-3102 may optimally support fecal and coat quality.