A representative of a group of related Rhodococcus arsenic resistance plasmids was characterized, locating the resistance genes and regions influencing host range and controlling copy number. This information, together with identification of antibiotic resistance determinants to replace the arsenic marker was used to construct Rhodococcus-Escherichia coli positive selection shuttle plasmids which, compared with those previously constructed, had half the size, a much higher copy number, and chloramphenicol rather than arsenic as selectable marker.