Infection of the small intestine of humans with the parasitic protozoon Giardia lamblia may have an asymptomatic course, or it may produce acute or chronic diarrhoea. In order to establish if the different clinical outcome of giardiasis in children could be due, at least partially, to strain differences, 19 isolates from asymptomatic and symptomatic cases studied in Mexico City were cultured under axenic conditions and the isoenzyme electrophoretic patterns of 10 different enzymes were compared. Strains from carriers and from symptomatic cases of giardiasis were equally amenable to isolation and axenization. Isoenzyme electrophoresis demonstrated remarkable homogeneity in 7 enzyme patterns for all 19 isolates, except for phosphoglucomutase, for which 3 different zymodemes were found. Therefore, these isolates of G. lamblia, obtained from a single geographical location, tended to be genetically homogeneous. In addition, there were no consistent zymodeme differences between isolates from symptomatic and asymptomatic human infections.