In polyandrous mating systems, females maintain the opportunity to bias male fertilization success after mating in a process known as cryptic female choice. Mechanisms of cryptic female choice have been described both in internal and external fertilizers, and may affect fertilization processes at different stages before, during and after fertilization. While in internal fertilizers, females have substantial control over sperm storage and fertilization, in external fertilizers, female control is limited. A key factor proposed to mediate cryptic female choice is the fluid surrounding the eggs, the ovarian fluid, as it may directly affect sperm performance. Here, we studied the role of ovarian fluid in post-mating sexual selection using the zebrafish, Danio rerio. Firstly, we assessed how ovarian fluid affects sperm swimming performance compared to freshwater. We focussed on sperm motility, velocity, swimming trajectory and longevity, all traits associated with competitive fertilization success in externally fertilizing fish. In a second step, we used a North Carolina II design to explore female, male, and female x male effects by testing sperm motility of two males in the ovarian fluid of two females in a total of eleven blocks. Our results suggest that the ovarian fluid affects sperm performance differently from freshwater. Specifically, sperm velocity, motility and longevity were higher in the ovarian fluid than in freshwater, whereas sperm linearity and beat cross frequency showed the opposite pattern. Moreover, these effects varied according to male and female identities, supporting the potential for cryptic female choice mediated by ovarian fluid in this species.