Abstract Six flatfish species were collected by trawling at five locations in the inner and middle Oslofjord between February and December in 2013. In total 386 individuals were sampled. The purpose of this investigation was to identify the infection of Hysterothylacium aduncum and Cucullanus heterochrous. Only three species contained these parasites: American plaice (Hippoglossoides platessoides), European plaice (Pleuronectes platessa) and Witch flounder (Glyptocephalus cynoglossus). Also the condition, age distribution and weight-length relationship was measured on these fish species. There was a geographical difference in the percentage of infected fish between the middle and inner part of the Oslofjord, as most nematodes were found in the middle part of the fjord. American plaice was the most abundant species and also the most infected specie. Two decades ago this species was not a known host, but during the last years this species has become an important host in the fjord. Hysterothylacium aduncum infected all three fish species with large abundances in the middle Oslofjord. It seems that H. aduncum has been in an increasing trend over the last two decades. American plaice starts to mature at 2 and 3 years, and these age groups were most infected. Females were more infected than males. There was no correlation between the length of the nematodes and the condition of the host. Cucullanus heterochrous was found in the inner Oslofjord, and this is only the second time this nematode has been reported in the inner part of the fjord, and both times it was found in European plaice and American.