Three phenotypes of phosphoglucose isomerase (PGI) from the Japanese quail erythrocytes were observed by horizontal starch gel electrophoresis. Population and family data from one laboratory population of quail was consistent with the theory that PGI polymorphism was controlled by two codominant, autosomal alleles designated PGIF and PGIS with gene frequency values 0.25 and 0.75, respectively. The study supported the earlier view that the Japanese quail is highly polymorphic with regard to biochemical variation.