Host-vector systems have been developed for gene cloning in the metabolically versatile bacterial genus Pseudomonas. They comprise restriction-negative host strains of Pseudomonas aeruginosa and P. putida and new cloning vectors derived from the high-copy-number, broad-host-range plasmid RSF1010, which are stably maintained in a wide range of Gram-negative bacteria. These plasmids contain EcoRI, SstI, HindIII, XmaI, XhoI, SalI, BamHI, and ClaI insertion sites. All cloning sites, except for BamHI and ClaI, are located within antibiotic-resistance genes' insertional inactivation of these genes during hybrid plasmid formation provides a readily scored phenotypic change for the rapid identification of bacterial clones carrying such hybrids. One of the new vector plasmids is a cosmid that may be used for the selective cloning of large DNA fragments by in vitro lambda packaging. An analogous series of vectors that are defective in their plasmid-mobilization function, and that exhibit a degree of biological containment comparable to that of current Escherichia coli vector plasmids, are also described.