Xanthurenic acid (XA) has been implicated as an inducer in vivo of exflagellation in Plasmodium spp. Consequently, the development of Plasmodium gallinaceum was assessed in a white-eye mosquito strain, kh^w, of Aedes aegypti (L.), which is deficient in XA because of a mutation of the gene encoding the enzyme kynurenine hydroxylase, and in a transformed line of kh^w mosquitoes that carry the wild-type cn gene of Drosophila melanogaster Meigen and express a functional enzyme necessary for XA production. Although XA was not detectable in kh^w mosquitoes by using high-pressure liquid chromatography with electrochemical detection, parasites were able to develop. Transformed kh^w mosquitoes failed to consistently support parasite development at higher prevalences and mean intensities than did the nontransformed kh^w lines, even though XA was detectable. These data suggest that factors other than XA may play a role in initiating Plasmodium development in vivo.