Isoenzyme analysis was conducted on the tachyzoite stage of 35 Toxoplasma gondii isolates. Fifteen enzyme systems were studied after isoelectrofocusing of tachyzoite extracts in polyacrylamide or agarose gels. Six enzyme systems showed variable electrophoretic patterns: aspartate aminotransferase (EC 2.6.1.1), glutathione reductase (EC 1.6.4.2), glucose phosphate isomerase (EC 5.3.1.9), amylase (EC 3.2.1.1), acid phosphatase (EC 3.1.3.2), and propionyl esterase. Their combination allows the description of 5 zymodemes among the 35 T. gondii isolates. Zymodeme 1 involves 6 isolates that are highly pathogenic to mice and for which oocysts could not be obtained. Isolates belonging to zymodemes 2, 3, and 4 are less pathogenic to mice and produced oocysts. Zymodeme 5 involves only 1 isolate, which was highly pathogenic to mice.