The host specificity of Marteilia pararefringens is under discussion after its suggested reseparation from the flat oyster pathogen M. refringens in 2018. In Norway, M. pararefringens has been detected in mussels Mytilus spp. sampled from several isolated, small heliothermic ponds (polls) that, at least on the western coast, inhabit some of the last reproducing flat oyster populations. M. refringens has never been detected in Norway. The polls have a limited water exchange, and their uniquely warm temperature can result in high M. pararefringens prevalence and infection intensities, representing unique sites to study the susceptibility of flat oysters to this parasite. We have sampled flat oysters and mussels from all known M. pararefringens sites along the Norwegian coast. All flat oysters and mussels were screened by histology and PCR. Furthermore, to study the potential effect of natural resistance breeding of local flat oysters subjected to repetetive M. pararefringens cycles, we deployed naïve flat oysters from a known Marteilia-free poll to Agapollen, where the parasite has had consistent infection cycles since its discovery in 2016. Naïve mussels were deployed simultaneously in 2 separate years to verify that the flat oysters were subjected to at least 2 transmission cycles. M. pararefringens was not detected in any flat oysters in any poll, local or naïve, despite presence in the mussel populations. Our results show that the European flat oyster Ostrea edulis is not a susceptible host for M. pararefringens.