The conjugative transposon Tn916 and a derivative Tn916 delta E was transferred from Bacillus subtilis into Clostridium difficile CD37 by filter mating. All the C. difficile transconjugants appeared to contain one copy of the transposon integrated into the same position in the genome. Transposition from the original site of integration was not observed. Like Tn916 the transferable tetracycline resistance determinant (Tc-CD) of C. difficile has a preferred site of integration in C. difficile and is homologous with Tn916 along the whole length of Tn916. However comparisons of the distribution of TaqI and Sau3AI sites in the homologous regions of the two elements did not demonstrate any hybridizing fragments in common.