Esterases of 42 strains of L. malonatica, L. amalonatica and Citrobacter were analyzed by horizontal slab electrophoresis in polyacrylamide-agarose gel using several synthetic substrates. On the basis of esterase zymograms a distinctive pattern was established for each of the 3 spp. L. malonatica was characterized by 2 major bands: 1 hydrolyzing acetate esters but not butyrate esters; and the other hydrolyzing .alpha.-naphthyl acetate and reacting weakly with .alpha.-naphthyl butyrate and .beta.-naphthyl acetate. L. amalonatica showed 1 prominent band that hydrolyzed .alpha.-naphthyl esters and reacted weakly with .beta.-naphthyl esters. Citrobacter strains showed 1 major band that hydrolyzed .alpha.-naphthyl esters and appeared slightly more active towards .beta.-naphthyl esters than that of L. amalonatica. Considerable variations in electrophoretic mobility were observed among Citrobacter strains. L. amalonatica was less variable. In addition, 1 minor anodal band reacting with .beta.-naphthyl acetate was observed in L. malonatica and Citrobacter. The relative molecular sizes of the major esterase bands were determined by disc electrophoresis with gels of different acrylamide concentrations. The molecular size of the major band of Citrobacter appeared to be smaller than that of the corresponding esterase band of L. amalonatica.