Sealworms, Pseudoterranova decipiens, are found in several fish species, and generally have a high prevalence in cod (Gadus morhua) and sculpins (Myoxocephalus scorpius). In the outer Oslofjord the final host is common seals (Phoca vitulina Linne, 1758), and here there are two colonies of these seals: a small population of 150 – 400 individuals in the outer Hvaler skerries in Norway and a large population of more than 2000 individuals in the Koster archipelago in Sweden. A feature of the sealworm life cycle in these habitats is that sculpins (Myoxocephalus scorpius) act as an important transmitter from invertebrates to cod. Heavily infested seals have eaten cod which have consumed heavily infected sculpins. The purpose of this project was to compare the sculpins found from Hvaler and Koster in order to investigate the effect of the number of common seals on the infection levels in sculpins.107 sculpins caught from the Hvaler Islands were investigated in 2009 and 161 sculpins from the Koster archipelago in the 1990’s. Females constituted slightly less than 70 % of the samples and had a significant different age structure than males. While the females were between 1.5 and 4.5 years old, the males achieved one year less: 1.5 – 3.5 years. All fish were between 12 and 29 cm. Females were approx. 5 cm longer than males.In Hvaler around 25 % of the sculpins were infested by sealworms and the number of sealworms per fish varied between 0 and 4. On average, the number of sealworms per investigated fish (abundance) was 0.4 in Hvaler, and the number of sealworms per infested fish (intensity) was 1.6. In Koster around. 85 % of the sculpins were infested by sealworms and the number of sealworm per fish varied between 0 and 34 with the exception of 102 sealworms in one female. On average, the number of sealworms per investigated fish was 6.4, and the number of sealworms per infested fish (intensity) was 7.5.There was no correlation between the number of sealworm and weight of the fish. In the Hvaler Islands, with few common seals, the number of sealworms per fish followed a Poisson distribution. In the Koster Islands with many common seals, the number of sealworms per fish had a large deviation from the Poisson distribution. In addition to the abundance of seals, the diet of sculpins may contribute to the observed differences in the distribution of sealworms in sculpins in the two areas.