Groups of Aedes triseriatus (Say) were reared either as nutritionally deprived (two regimens) or well fed (one regimen) throughout larval development, and the vector competence of resulting small, normal, and large females was assessed for La Crosse virus. When fed a high dose of virus (4.6 log10/0.025 ml in Vero cell culture), 90% of small Ae. triseriatus females transmitted La Crosse virus to suckling mice compared with 70% of normal and 42% of large females. Among small females, 100% had disseminated infections as did 86% of normal females, whereas only 69% of large females had disseminated infections. All females had infected mesenterons (midguts). When fed a low dose of virus (2.2 log10/0.025 ml in Vero cell culture) in a second experiment, 15% of small females transmitted compared with 0% of large females; 50% of small females developed disseminated infections compared with 16% of large females. mesenteronal infection occurred in 70% of small but only 32% of large females. Electron microscopy of mesenteronal tissues from large and small females revealed physical differences in the basement membranes (basal laminae). The mesenterons of small females had 3-6 laminae (mean thickness of the basement membrane = 0.14 microns) compared with 9-16 laminae (mean thickness of the basement membrane = 0.24 microns) in large females. These morphological differences indicated that the mesenteronal escape barrier, which accounted for the difference in the percentage of small and large females with disseminated infections, may be, in part, a physical barrier that was modified by nutritional deprivation in the larval instars.