SUMMARY Leucocytes and erythrocytes from 92 unrelated mongrel dogs were tested for the occurrence of electrophoretically detectable isoenzyme variations in 15 enzyme systems believed to be determined by 21 different genetic loci. Three loci (PGM3, D-TO, and PGM2) exhibited polymorphism and four others (G6PD, PGM1, C-GOT, and M-MDH) have each shown a variant, occurring only once in the present dog population. The other loci tested (ADA, AK, GPI, M-GOT, C-IDH, M-IDH, LDH A, LDH B, C-MDH, Pep A, Pep B, Pep C, PGD, and PGK) did not show variation under the conditions of electrophoreses employed.