On the basis of the evidence that insect fungivory has the potential to affect fungal reproductive fitness, we investigated the effects of two specialist ciid beetles (Octotemnus glabriculus and Cis boleti) on the reproductive potential of their host fungus, Coriolus versicolor. We found, from field data, a negative correlation between the number of individuals of O. glabriculus inhabiting C. versicolor fruit bodies and the percentage of the fungal spore-producing surface (hymenium) that was functional. By contrast, the number of C. boleti inhabiting C. versicolor fruit bodies did not correlate with the percentage of functional hymenium. Experimentally, O. glabriculus and C. boleti reduced the reproductive potential of C. versicolor by 58% and 30%, respectively, whereas the combined trophic activity of both beetles caused a reduction of 64%. This latter effect was not significantly different from that caused by O. glabriculus alone. These findings disagree with previous assertions that insect fungivory on fruit bodies has only neutral effects on fungal fitness. We conclude that in the short-term, fungivory by ciids significantly decreases the area of functional hymenium of C. versicolor and is likely to reduce fungal reproductive fitness. Within this perspective the evolution of certain fungal characteristics (i.e. chemical composition, consistency and phenology) can be interpreted as being driven by fungivory.