When Moraxella plasmid pUO1 encoding haloacetate dehalogenase and mercury resistance coexisted with IncP-1 plasmid RP4 in Pseudomonas sp., genetic exchange between the plasmids often occurred, probably by site-specific recombination. The recombinant plasmids obtained were classified into four groups on the basis of phenotype. Representative plasmids for each group were analyzed for DNA composition and function, and the mechanism for the formation of these plasmids was sought. They were inherited stably in Escherichia coli and a Pseudomonas sp.