Herbivorous insects often feed on pathogen-infected plants in nature, and it is likely that pathogen infection alters host plant quality. We documented the effects of plant infection by a widespread plant virus on host plant quality for a generalist insect herbivore and tested the hypothesis that these effects vary among plant genotypes. We found that infection by Cucumber mosaic virus (CMV) altered the host plant quality of Mimulus guttatus (Phrymaceae) for meadow spittlebugs, Philaenus spumarius (Hemiptera: Cercopidae). The effects of CMV infection on host plant quality, however, varied among full-sib M. guttatus families, suggesting that these effects vary among plant genotypes. In most full-sib families, CMV infection either had no effect on host plant quality or increased host plant quality as measured by spittlebug size and development time. In a few M. guttatus families, however, CMV infection decreased host plant quality. There was no relationship between the effect of CMV infection on plant growth and the subsequent effect of CMV infection on spittlebug performance, suggesting that broad changes in host plant performance (e.g., growth rate) were not responsible for the effects of CMV infection on host plant quality for spittlebugs. We suggest that future studies of the effects of pathogen infection of plants on insect herbivores consider variation among plant genotypes in the mechanisms and ecological consequences associated with these effects.