Parasitological analysis of sockeye salmon fry from Lake Azabach'e has shown reliable differences in the infection between aboriginal and migratory in the lake fry with the nematode Cystidicola farionis from the swim bladder, that is connected with differences in the nature of their feeding and ecology. It was found out that the infection of migratory fry occurs only in the lake during feeding and wintering periods, one-summer-old fry 35 to 57 mm in length being primarily infected. Quantitative criteria (the abundance of immature nematodes of C. farionis, the type of feeding) were ascertained, which in combination with scale characters differentiate 95.9 per cent of fry into aboriginal and migratory.