Genetic studies of indophenol oxldase (IPO), phosphoglucose isomerase (PGI), and peroxidase (PER) enzyme systems in ryegrass showed that electrophoretically detectable variation in each system is governed by asingle Mendelian locus with codominant alleles. The three loci are inherited independently of each other. Aberrant segregation ratios at the PER locus may result from involvement of PER allozymes in the incompatibility system of ryegrass.