Clones of 32 strains of Trichomonas vaginalis isolated from patients attending a venereal diseases clinic were compared among themselves and with authentic Pentatrichomonas hominis on the basis of their isoenzyme patterns for eight enzymes by thin-layer starch-gel electrophoresis. The enzymes examined were: glucose phosphate isomerase (GPI); phosphoglucomutase (PGM); malic enzyme (NADP+) (ME); hexokinase (HK); malate dehydrogenase (NAD+) (MDH); glucose-6-phosphate dehydrogenase (G6PD); aldolase (ALD); and lactate dehydrogenase (LDH). From the isoenzyme patterns of four enzymes (LDH, MDH, HK, and GPI) the strains of T vaginalis could be divided clearly into five groups. PGM showed differences in only one strain, while two other enzyme patterns (ME and ALD) were the same for all the strains of T vaginalis tested. All isolates were clearly distinguishable from P hominis. Although G6PD patterns were not sharp some differences were evident among T vaginalis strains.