Cryptic female choice (CFC), a post-mating version of sexual selection, enables females to bias fertilization outcomes towards certain males in a non-random fashion. Here we report our investigations on several potentially important intersexual postcopulatory gametic interactions in a population of chinook salmon (Oncorhynchus tshawytscha): the effect of female ovarian fluid (OF) on the behaviour of spermatozoa during fertilisation, and the effects of MHC variability, multilocus heterozygosity (as an index of male quality) and female-male genetic relatedness on sperm behaviour and and male fertilisation success when there is sperm competition in the presence of that ovarian fluid. To do this, we conducted a series of in vitro competitive fertilisation experiments and found that, when ejaculates from two males are competing for access to a single female’s unfertilised eggs, fertilisation success was significantly biased toward the male whose sperm swam fastest in the female’s ovarian fluid. Fertilization success was higher for males more similar to the focal female at both microsatellites, SNPs and the MHC class II than their competitor, perhaps indicating that the MHC class II and other genetic factors mediate sperm-egg interactions. Embryo survival—a measure of fitness—was also positively correlated with both sperm swimming speed in ovarian fluid and male multilocus heterozygosity, providing novel evidence that cryptic female choice is adaptive for the female, enhancing the early survival of her offspring and potentially influencing her fitness.