SUMMARYPlasmids RP4 (IncP) and R300B (IncQ) (mobilized by RP4) were transferred to Methylomonas albus from Escherichia coli donors at frequencies of 2 × 10 3 and 1-10 3 per donor. BY contrast, plasmids pS-a (IncW) and the IncP plasmids R68.45, R751 and derivatives carrying bacteriophage Mu and/or transposons were transferred to M. albus from E. coli at frequencies of 10 7-10-8 per donor. M. albus transconjugants carrying R68.45, R751 and their derivatives were isolated and used as donors in matings with either other strains of M. albus or E. coli. Plasmids carrying bacteriophage Mu were transferred to E. coli at a frequency of 10 5-10-6 per donor and 10 2-10 3 per donor for those not carrying Mu. Transfer to M. albus recipients was at a frequency of 10 3-10 -4 per donor regardless of the presence or absence of Mu, indicating the absence of a suicide effect. Plasmids carrying Tn5 were found in M. albus to determine simultaneous kanamycin (Km) and streptomycin (Sm) resistance.