1. Electrophoresis of human tissue extracts has indicated a specific pattern in fibroblasts for the enzyme triose phosphate isomerase. In other human tissue extracts only two zones I and II of activity were seen. In fibroblasts extracts a more anodic III band was also visible. 2. Electrophoretic band II was separated from bands I and III. From a study of dissociation by concentrated urea followed by electrophoresis, it was demonstrated that isozyme II is a hybrid between isozymes I and III. An immunological difference was found between isozyme I and II using an antiserum obtained by immunizing rabbits against purified isozyme I. 3. An evolutionary study showed that other mammals and monkeys are lacking the fast bands II and III, while five species of Hominoids (including man) possess them. 4. The tentative conclusion was drawn that isozyme III is made of twoβ subunits, isozyme I of twoα subunits and isozyme II is anαβ hybrid. The hypothesis is presented of a gene duplication having appeared at the time of differentiation of Hominoids.