During nitrogen starvation, a 20- to 250-fold increase in specific urease activity was observed in extracts of P. aeruginosa, P. fluorescens, Hydrogenomonas, M. denitrificans, M. cerificans and B. megaterium. In contrast to these species, high levels of urease were observed in P. vulgaris strains and in S. ureae under all growth conditions. No urease was detectable in strains of E. coli, S. marcescens and B. polymyxa, regardless of growth conditions. Incubated in the absence of an exogenous nitrogen source, the specific urease activity increased during a period of 10 to 20 h in P. aeruginosa, Hydrogenomonas and M. denitrificans. Phosphate starvation did not significantly effect urease formation in these strains. The increase in specific urease activity was found to be repressed by exogenous nitrogen sources, including urea. Inhibition by chloramphenicol, other inhibitors, and by the lack of oxygen or fructose, indicated that a derepressive urease formation may occur in these strains. The involvement of traces of urea possibly released from endogenous sources during starvation is discussed.