Isoenzyme patterns of 63 isolates of Trichomonas vaginalis obtained in Vancouver were evaluated by use of thin-layer starch-gel electrophoresis. We attempted to use eight enzymes, but only four gave reproducible and interpretable results. There were four patterns with malic enzyme, two with malate dehydrogenase, one with hexokinase, and four with lactate dehydrogenase. The isoenzyme patterns of the 63 isolates were classified into 15 groups, but 49 (78%) fell into five groups and 14 (22%) fell into ten groups. There was no obvious correlation between groups and magnitude of symptoms and signs, past history of trichomoniasis, or likelihood of treatment failure. Results were consistent for isolates obtained from the same patient on different days. This system will allow differentiation of isolates into groups, a procedure that could be useful. However, groups do not appear to correlate with clinical or historical features or with outcome of treatment.