Two of the five lactate dehydrogenase (LDH) subunit loci known to exist in salmonid fishes were obtained in heterozygous condition in the hybrid of lake trout (Salvelinus namaycush Walbaum) X brook trout (S. fontinalis Mitchill) and linkage tests were performed. These loci (and subunits), designated A and B, are both regarded as being duplicates of and therefore homologous to the B locus of higher vertebrates. In first backcrosses of doubly heterozygous males to homozygous brook trout females there was a significant deficiency of parental combinations among the offspring. However, when the doubly heterozygous parents were female, there were equal numbers of the four backcross progeny types produced, indicating independent assortment. Second backcrosses to brook trout homozygotes produced similar results. Among first backcross families there was a range of values of parental combinations of from 13.5–30.0%, while among second backcrosses this value ranged from 1.0–66.0%. The latter value is an exceptional case in which parental combinations were in excess. The data thus do not indicate linkage of the A and B genes. Possible causes of the disturbed joint segregation in males are discussed.