Abstract 1. 1. Several multiple forms of phosphofructokinase have been found in striated muscle, cardiac muscle, brain and liver of animals from the Phylum Chordata. 2. 2. PFK forms of lower electrophoretic mobility occur in mammalian striated muscle, while those of liver show the greatest electrophoretic mobility. 3. 3. Mammalian PFK of striated muscle in most cases appears simultaneously in cardiac muscle and in brain of the same animals. Specific forms of PFK in brain and in cardiac muscle are not so frequent, but are more frequent in brain than in cardiac muscle. Also, it is very frequent to find several isoenzymes in brain and in cardiac muscle of the lower classes of vertebrates. Among the Mammals the liver enzyme seldom coexists in other tissues of the same animal.