ABSTRACT: Fluctuating asymmetry (FA), defined as random deviations from perfect bilateral symmetry, is assumed to reflect developmental instability. FA is predicted to increase in response to environmental stress, including parasite infection. In addition, based on theory we predict a higher FA in sexually selected traits, due to their greater sensitivity to stress. We investigated the relationships between FA, parasitism and reproductive fitness in 2 species of gammarid crustacean, incorporating both sexual and non-sexual traits. We tested the hypothesis that gammarids infected by vertically transmitted Microsporidia will display higher levels of FA than those infected by horizontally transmitted trematodes, because vertically transmitted Microsporidia can be present at the earliest stages of host development. We found little evidence for a relationship between FA and fecundity in Gammarus spp.; however, egg diameter for infected female Gammarus duebeni was significantly smaller than uninfected female G. duebeni. FA was not correlated with brood size in females or with sperm number in males. In contrast to our prediction, we report a lower relative FA in response to sexual traits than non-sexual traits. However, FA in sexual traits was found to be higher in males than females, supporting the theory that sexual selection leads to increased FA. Additionally, we report a negative correlation between FA and both trematode (Podocotyle atomon) and PCR-positive microsporidian (Nosema granulosis and Dictyocoela duebenum) infections and interpret these results in the context of the parasites’ transmissionstrategies. FA in G. duebeni and G. zaddachi appears to associate with trematode and microsporidian presence, although reproductive fitness is less altered by infection.