Seroconversion is well known to occur in the hosts of mosquitoes. In previous studies, the process of seroconversion has been shown to have negative effects on the fitness of other arthropods. My research has shown that seroconversion also negatively affects mosquitoes, reducing the volume of their blood meals, the number of eggs they can produce, and the number of those eggs that are viable. Seroconversion was confirmed by western blot analysis. The effects were seen with all four species used in this study (Aedes aegypti, Aedes albopictus, Anopheles stephensi, and Anopheles gambiae). Some level of cross reactivity was also observed as members of the same genus negatively affected each other. When the experiment was repeated using B-cell knockout mice (mice unable to seroconvert), there was no observed fitness reduction. This result confirms the hypothesis that host seroconversion is necessary for the observed fitness reduction to occur.