The incidence of larval nematodes, Porrocaecum decipiens, in fillets from three size-groups (scrod, market and steak) of Atlantic cod, Gadus callarias, has been analyzed. The analysis was based on about 73,000 fillets from cod caught between 1946 and 1956 in about 20 areas off the southern Canadian mainland.Geographic variation in the percentage of fillets infected was found in all size-groups of cod. For market cod the incidence ranged from a low of 6% on the offshore Nova Scotian Banks to a high of 35 to 91% in the southwestern Gulf of St. Lawrence. Intermediate values of 14% and 22% were observed respectively in the inshore fisheries of western Nova Scotia and Cape Breton Island. Similar patterns of geographic variation were present in scrod and steak cod. The main geographic differences in incidence were related to the distribution of the harbour seal (Phoca vitulina), the harp seal (Phoca groenlandica) and the gray seal (Halichoerus grypus).Incidences of infection and the number of nematodes per fillet usually varied directly with the size of the cod. The number of nematodes per pound of fillet was usually inversely related to the size of cod. It was concluded that cod could become infected throughout life and that the highest rate of infection occurred in small cod.Local variations, as distinct from broad geographic variations, were observed in the areas of Lockeport, of Cape Breton Island and of the southwestern Gulf of St. Lawrence. In these areas, the cod caught closest to shore were usually the most heavily infected.Seasonal variations in incidence occurred in the Cape Breton and Lockeport fisheries. The highest incidences occurred in the spring and fall in Cape Breton cod and in the summer in Lockeport cod. The variation in the Cape Breton fishery was attributed to seasonal migrations of the cod comprising the fishery. Increased use by fishermen of shallow inshore areas in the summer accounted for the higher summer incidences at Lockeport.Annual fluctuations in incidence were great in all areas. A trend of declining incidences was noted in the southwestern Gulf of St. Lawrence where the incidence dropped from 91% in 1946 to 35% in 1956. This decline was apparently related to extensive contemporary changes in the cod fishery in the Gulf of St. Lawrence.