Isolates of Blastocystis hominis derived from patients with intestinal symptoms and from healthy carriers were cultured in vitro and isoenzyme patterns of hexokinase (E.C. 2.7.1.1), phosphoglucomutase (E.C. 2.7.5.1) and glucose phosphate isomerase (E.C. 5.3.1.9) were investigated to find evidence for pathogenic and non-pathogenic subspecies of B. hominis. For this purpose we examined 2000 patients of the out-patient department of the Institute for Tropical Medicine in Tübingen. From these, we obtained 232 B. hominis patient isolates; 119 isolates could be tested for the three isoenzymes. Blastocystis hominis possesses 5 patterns for hexokinase, 11 for phosphoglucomutase and 35 for glucose phosphate isomerase, showing that B. hominis is highly polymorphic. However, there was no correlation between isoenzyme patterns and disease of patients.