Transmission of nuclear polyhedrosis virus (AgNPV) in uniform- and mixed-aged populations of velvetbean caterpillar (VBC),Anticarsia gemmatalisHübner, larvae artificially established on field-caged soybeans was examined. In a uniform-aged VBC population, transmission of AgNPV from primary infected to uninfected VBC was generally low. Larval mortality from AgNPV was highest in treatments with highest density of primary infected larvae, but did not differ with size at death of primary infected larvae. Transmission in mixed-age populations was greater than in uniform-aged populations when primary infected larvae were of the same age structure as the uninfected population or all the age of the oldest uninfected larvae in the population. Virus concentration on foliage after pupation of survivors in both uniform- and mixed-aged populations was high, but little viral activity remained on foliage 1 mo later. Virus concentrations in soil in both tests were generally low immediately after pupation but were much higher 1 mo later.