A model is proposed to describe the accumulation of sealworm parasites (Pseudoterranova decipiens, Nematoda, Ascaridoidae) in Atlantic cod (Gadus morhua). The model predicts the total amount of food eaten by a fish to reach its size. In young fish growth is linear in time and average parasite burdens are predicted to increase with the cube of the average length of the fish divided by its growth rate. An analysis of worm burdens in cod caught by the commercial fishery in the Firth of Clyde (Strathclyde, Scotland) between 1964 and 1970 illustrates how average parasite abundances at age, length, or fixed time of the year can be compared. It is concluded that exposure did not vary significantly over these years. Extensive apparent inter-annual variations in worm burdens of cod newly recruited to the fishery are well explained by differences in the age composition of cod catches.