An enzymatic parameter was examined with a view to providing a supplementary criterion to individualize separate geographical populations of a single fish species, Disostichus eleginoides (Notothenioidei). An acid phosphatase of specimens fished from the Crozet and Kerguelen islands in February 1982 and February 1983 was studied and compared between populations. The enzyme was extracted from the white back-muscle of fresh fish and proved to be very similar in both groups, except that their K m s differed slightly. The optimum temperature for in vitro enzyme activity was 30°C, optimal pH was 5; the active purified polypeptides had a molecular mass of 55 kdaltons and exhibited very low substrate-specificity.