Two mutants of Zymomonas mobilis defective in phosphatidylethanolamine N-methyltransferase have been isolated. One mutant was completely defective in all of the three N-methyltransferase activities dependent on phosphatidylethanolamine, phosphatidyl-N-monomethylethanolamine and phosphatidyl-N, N-dimethylethanolamine, respectively. The other was a leaky mutant in which the three enzyme activities were all equally reduced to 3.4-6.7% of those in the parent strain. Two revertants were derived from the leaky mutant, and in both all three enzyme activities were restored to nearly the same levels as those in the wild-type strain.