SummaryUsing starch gel electrophoresis, polymorphism of phosphohexose isomerase (PHI) was found in haemolysed swine erythrocytes.Three different phenotypes A, AB and B were observed. They are determined by two autosomal codominant allelesAandB.Homozygous types have three fractions; a strong middle band flanked by two weaker ones. The heterozygous types have five fractions, three strong bands in the middle with weaker anodic and catodic bands.Segregation data in the family material support this hypothesis. The age does not influence the pattern of the PHI types.