Grasshoppers (Orthoptera: Acrididae) are hosts to many internal parasites, including nematodes. However, the effects of nematode parasitism on grasshopper fecundity and how these effects vary with population density are unclear. I report on the fecundity of Melanoplus dawsoni (Scudder) infected and uninfected with mermithid nematodes in northern Wisconsin from 2002 to 2005. Each year grasshoppers were stocked over a range of five densities into field enclosures. Fecundity, body size, and nematode prevalence were measured for female grasshoppers. Nematode prevalence was moderately high, ranging from 15% in 2003 to 37% in 2004. Fecundity was measured by examining grasshopper reproductive tracts. On average, past reproductive activity (number of eggs laid) and current reproductive activity (number of eggs forming) of parasitized grasshoppers were reduced by 40% and 48%, respectively. Interestingly, the reduction in fecundity was less for parasitized females in the low-density treatments (25%–50% of field density) than for those in the moderate- and high-density treatments, suggesting that grasshoppers can compensate somewhat for negative effects of parasites on fecundity when per-capita resources are high. No difference in hind-femur length between parasitized and unparasitized females was observed, indicating that nematode infection did not affect grasshopper body size.