Various mutants (lip) of Pseudomonas aeruginosa PAO 2302 that lacked extracellular lipase activity were isolated. They were selected on a calcium-triolein agar. The phenotypic characteristics of two of these mutants suggested that they were defective in the gene coding for lipase: both lip mutants produced no lipase in liquid- and on solid medium. They were nonpleiotropic with regard to various other exoproducts. None of the mutants released any putatively cell-bound lipase after treatment of cells with Triton X-100 or alginate. The electrophoretic protein- and LPS-profiles of outer membranes derived from lip mutants and the parental strain were identical. The lip locus was mapped on the chromosome of P. aeruginosa PAO 1 by FP5- and R68. 45-mediated crossings and by transduction with phage G101. The lip locus was cotransduced with pyrF only (60%) indicating a map position at about 57 min. The lipase gene was cloned on a 3.1 kb SalI fragment using vector pKT248. The newly constructed plasmid was able to complement the lipase deficiency of the two lip mutants of P. aeruginosa.