Phosphatidylserine decarboxylase catalyzes the last step in the pathway leading to phosphatidylethanolamine, the principal membrane lipid of E. coli. Mutants of E. coli have now been isolated in which this enzyme is theramolabile. The structural gene for phosphatidylserine decarboxylase (psd gene) is closely linked to the pur A locus at about 83 min on the standard map of the E. coli chromosome. When a mutant with thermolabile decarboxylase is incubated at 42 degrees, growth ceases, but only after a substantial fraction (20-40%) of the total phospholipid of the cell has been replaced by phosphatidylserine. Examination of such mutants with altered content of phospholipids may shed light on the role of specific phospholipids in membrane function.