Plasmid PUO1, which is present in a fluoroacetate-assimilating strain of Moraxella sp., specifies two kinds of haloacetate dehalogenases, H-1 and H-2, and mercury resistance. The H-1 and H-2 genes were cloned onto plasmid pBR322 in Escherichia coli. The analysis of the cloned DNA fragments revealed the loci of both genes on the pUO1 map and also the direction of transcription. Two different hybrid pBR322s containing the H-1 gene were maintained stably in E. coli, while three hybrids containing the H-2 gene were so unstable that they readily disappeared or changed into various deletion mutants. It was suggested that the instability of the H-2 hybrid plasmids might be caused by a certain DNA structure adjacent to the H-2 gene which might be concerned in the deletion of the H-2 gene from pUO1.