A tetracycline resistance (Tc') determinant from Closfridium &Bile strain 630 was cloned into the Escherichia coli plasmid vector pUC13. The resulting plasmid pPPM20, containing an insert of 3.4 kbp, was mapped and a 1.1 kbp SucI-Hind111 fragment wholly within the Tcr gene was identified. Dot-blot hybridization studies with the 1.1 kbp fragment showed that the Tcr gene belonged to hybridization class M. Tcr could be transferred between C. &mile strains and to Bacillus subtilis at a frequency of lo-' per donor cell. The element could be returned from B. subtilis to C. &@cik at a frequency of per donor cell. This is the first demonstration of C. &Wile acting as a recipient in intergeneric crosses. DNA from C. dzBcile transconjugants digested with EcoRV always has two hybridizing fragments of 9.5 and 11.0 kbp when probed with pPPM20. DNA from B. subtiZis transconjugants digested with EcoRV produced one hybridizing band of variable size when probed with pPPM20. The behaviour of the element was reminiscent of the conjugative transposons. Therefore we compared the element to the conjugative transpomn Tn916. The Him11 restriction maps of the two elements Mered and no hybridization was detected to oligonucleotides directed to the ends of Tn916. However, the elements do have some sequence homology, detected by hybridization analysis.