Increasing global plastic production along with plastic littering leads to accumulation of microplastic particles in aquatic environments. Microplastics are detected in aquatic habitats worldwide, whereby microplastic fibers belong to the predominant plastic types. As early life stages of aquatic organisms are predicted to be especially vulnerable to microplastic pollution, we tested the impact of polyester fiber presence in the water during in-vitro fertilization and embryo development of three-spined sticklebacks (Gasterosteus aculeatus). Egg clutches (n=6) were split and one half fertilized in water containing pristine polyester fibers while the other eggs served as control treatment without fibers. Fiber concentration was set at 10 000 fibers per liter and maintained with water exchange throughout the entire experimental period. Fibers were kept in suspension by continuous agitation of the breeding bowls. Observation with a dissection microscope revealed that some polyester fibers stuck to the outside of the eggs in the fiber treatments. Yet, no significant difference in fertilization, hatching or mortality rates were observed between the divided egg clutches. Furthermore, heart rate of embryos and morphological features of hatched larvae were not affected by fiber presence in the water. The results suggest that fertilization and early development of stickleback eggs are unaffected by pristine polyester fibers in the water column, even at fiber concentration above current environmental levels.