The paper presents data on the infection of fishes from the Gulf of Peter the Great with an A. simplex nematode. A total of 308 fishes of 19 species were examined for parasites. Of them 5 species were found to be free from nematodes. The highest infection rates were noted in fishes of the species Oncorhynchus keta, Hexagrammos octogrammus, and Theragra chalcogramma. Differences in the infection rates of fishes of the gulf were explained by the specific features of their vital activity and by the saturation of the environment with an invasive agent. Based on the infection of secondary intermediate hosts with the nematoda, it may be suggested that there are the populations of fishes whose representatives accomplish their life cycle under the conditions of the internal waters of the gulf. The infection rate of these fishes with the parasite is low. The altered situation--an increase in the invasion rate--was associated with the seasonal migration of fishes of these species from the open marine spaces to the cost of the Gulf of Peter the Great. The opposite values of the infection rates of deep-sea (herrings) and bottom-dwelling (bases, greenlings) fishes make it possible to assume that bottom-dwelling invertebrates function as the first intermediate hosts in the life cycle of the nematode.