Catalase isozymes were examined in a wide range of peach [Prunus persica (L.) Batsch] cultivars representing historical U.S. cultivars, commercial cultivars from numerous North American breeding programs, and the peach plant introduction (PI) collection. All historical peach cultivars from the United States and those released from commercial breeding programs were fixed for the slow (Cat l-2) allele, with the exception of `Belle of Georgia', `Honeyglo' nectarine, and various cultivars from the Univ. of Florida breeding program, which possessed a fast-migrating (Cat 1-l) allele in homozygous or heterozygous state. Polymorphism was revealed in the 51 peach PI clones examined, with allelic frequencies of 0.69 and 0.31 for the Cat l-2 and Cat l-1 alleles, respectively. Most PIs that originated directly from China were homozygous Cat l-l/Cat l-l, while most PI clones introduced from Europe were homozygous Cat l-2/Cat l-2. Examination of the catalase genotype of cultivars previously proposed as the possible male parent of `Belle of Georgia' (`Champion', `Early Crawford', `Late Crawford', `Oldmixion Free', and `Stump-the-World') revealed that none of these cultivars could have been the male parent of `Belle of Georgia'. Segregation data from various peach crosses was consistent with the hypothesis that catalase polymorphism could be explained by the presence of two alleles at a single locus.