The purpose of this study was to assess the carriage of Clostridium difficile by household pets to determine their potential as a reservoir of infection. The selective cycloserine-cefoxitin medium was used for C difficile isolation, and tissue culture used for detection of cytotoxin. Carriage of C difficile by household pets was found to be common (23%). The carriage tends to be transient and does not appear to be associated with gastrointestinal disease. Although carriage was higher in animals who had antecedent antibiotic treatment (31%) compared to those which had not (19%), the differences were not statistically significant. In most cases non-cytotoxigenic strains were isolated. Of the cytotoxigenic strains isolated at least one strain was pathogenic in a well documented animal model of human disease. Both cytotoxigenic and non-cytotoxigenic strains of C difficile could be isolated from the environment of the animals studied.