The effect of larval age on the progress of epizootics of a granulosis virus was examined in field populations of imported cabbageworm,Artogeia rapae(L.), in 1985 and 1986. An enzyme-linked immunosorbent assay (ELISA) was used to follow the progress of disease in three larval age classes beginning 2–3 d after treatment. On virus-treated cabbage, late second and early third instars were more liable to infection than were first and final instars. In laboratory experiments, potted cabbage plants were treated with virus in the same way that field plots had been, and leaves were fed to three age classes of larvae at three temperatures for 18–23 h. Fewer larvae developed disease when given access to leaves at 16°C than at 22 or 28°C. Fewer first instars became infected than third and fifth instars. Third instars were most likely to become infected at all temperatures. The data suggest that virus should be directed at second and third instars rather than at larvae at hatching.