A bank of 600 insertional mutants of Mycobacterium smegmatis was screened for mutants defective in stationary-phase survival. Of 74 mutants picked by the initial screen, 21 had stationary-phase survival defects and 7 of these were studied in more detail. In general, mutants survived stationary phase significantly less well in rich medium than under carbon-starvation conditions. In all cases the loss of viability in stationary phase was not complete even after prolonged incubation. All mutants showed an initial decrease in viability, during the first 40 d in stationary phase, followed by an increase in viable counts that returned viability close to the levels of the wild-type. Southern hybridization experiments showed that recovery of viability was not a consequence of precise excision or movement of the transposon. Two of the survival mutants differed from the wild-type in their colony morphology, and recovery of their viability in stationary phase was coincident with the return of wild-type colony morphology. It is possible that second-site suppressor mutations accumulate that alleviate the effects of the original mutation. For five of the mutants the DNA flanking the site of transposition was amplified by ligation-mediated PCR and sequenced to identify the disrupted locus. In each case, homologous genes were identified in the Mycobacterium tuberculosis genome, three of which have clearly predicted functions in M. tuberculosis as a penicillin-binding protein, in biotin biosynthesis and as a polyketide synthase. This is the first identification of genes implicated in the stationary-phase survival of mycobacteria.