Western tent caterpillar,Malacosoma californicum pluviale(Dyar), larvae were treated with nuclear polyhedrosis virus (NPV) in the laboratory to assess whether the presence of virus could potentially explain reduced fecundity observed in declining field populations of the host. In 1992, male and female pupal weights and female fecundity were reduced in survivors of virus treatment. In 1993, using the same viral dosage on individuals from a different population resulted in reduced pupal weight (and fecundity) in females only. Strong family effects were also detected for male and female pupal weights and fecundity in both years. Results from 1992 data showed a significant effect of virus treatment on fecundity when variability related to pupal weight was removed, suggesting that NPV could directly affect egg production. This pattern was not observed in 1993. These experiments suggest that NPV has the potential to cause reduced fecundity in field populations of the western tent caterpillar.