Breeding systems for species with cryptic lives are difficult to examine in nature based on purely observational studies. In this study, a genetic approach was used to investigate the breeding system of a population of a stream spawning salmonid, the European grayling (Thymallus thymallus), from a tributary in the Norwegian lake system, Lake Lesjaskogsvatnet. For this purpose, highly polymorphic microsatellite loci were used to genotype adult grayling sampled in 2008. Those genotypes were in turn used to assign parentage to > 800 grayling fry sampled in 2008, which were genotyped for the same loci. In order to achieve the best possible results, I applied two different methods of parentage assignment and compared their results. Despite differences in individual results, overall, both methods were consistent in their major findings. Both confirm an apparent polygynandrous grayling breeding system, that is, that grayling of both sexes mated successfully with more than one partner during spawning in 2008 in Lake Lesjaskogsvatnet. In addition, a large variation in reproductive success and a reproductive skew for both sexes were observed. However, this variance in individual reproductive success could not conclusively be explained, neither with body length nor with the timing of spawning migration.