The mosquito Armigeres subalbatus can encapsulate and kill > 80% of Brugia malayi microfilariae (mf) within 36 hr following ingestion. The cascade of biochemical events constituting this melanotic encapsulation response is also important in other mosquito biological events, including egg-chorion tanning. Certain biochemical entities, including a tyrosine precursor, are thought to be shared among these biological activities. Because of this purported tyrosine link, and because the blood meal both initiates egg development and is the source of mf, we evaluated the possibility that reproductive cost is incurred by the resistant host when undergoing a response to mf acquired in an infected blood meal. Mean time to oviposition was significantly longer for mosquitoes responding to parasites than for controls (77.7 versus 66.5 hr). Tyrosine levels in ovaries from infected mosquitoes were less than half those of controls at 24 and 48 hr, and were still significantly reduced at 72 hr following bloodfeeding. Ovary development, assessed via measurements and total protein content, also was delayed significantly in the experimental group, with ovary width and protein content never attaining levels found in control mosquitoes. Sections from 24-hr post-blood meal ovaries demonstrated that the normal processes of egg development, including vitelline accumulation, was drastically altered as well. The biological implications of these results are considered.