Winter survival of organisms has a crucial effect on their fitness in a seasonal environment. We tested whether overwinter survival of male water striders, Aquarius najas (De Geer) (Heteroptera: Gerridae), is associated with the time of season they leave the water for winter diapause. This was performed by comparing parameters of males leaving the water at the beginning of autumn and 1 month later. The results show significantly higher overwinter survival inmales of the early diapausing group. They were found to be larger than late diapausing individuals. Overall, overwinter survival was found to be associated with body length. The strength of immune defence measured as encapsulation response against nylon monofilament appeared to be stronger in the early than in the late diapausing group. Body length had an effect on the encapsulation rates of water striders, with larger males having a stronger encapsulation response. The amount of lipids was larger in males of the early diapausing group, and the relationship between body length and absolute lipid reserves was positive in both groups. However, a correlation between body reserves and body length was not found when lipid content was expressed as a proportion of total body weight. The results suggest that body length is the main variable linking overwinter survival and resistance against pathogens and parasitoids. This study shows that differences in resistance can explain differential winter survival of larger and smaller A. najas water striders.