Analysis of peroxidase isoenzymes in interspecific elm hybrids indicated that a close correlation exists between parental isoenzymes and those of the progeny. In some crosses, isoenzymes undergo mendelian segregation while in others segregation distortion is evident. Mechanisms proposed to account for the disturbed segregation ratios and unexpected phenotypes include crossing over between peroxidase loci, accumulation of lethal genetic deficiencies, and dominance of some peroxidase genes.