Comparisons of species richness between parasite component communities are often confounded by uneven sampling effort and the possibility that rare species have been missed from some component communities. The use of nonparametric estimators of species richness could potentially alleviate this problem by allowing the number of missing species to be extrapolated from the observed data. The performance of 3 estimators and their sensitivity to true species richness and the frequency of rare species, i.e., species with low prevalence, were tested using computer-simulated parasite communities. When the number of hosts examined in a sample is large, the observed species richness is an accurate estimate of true richness; no extrapolation is necessary even when rare species make up a large part of the community. At small sample sizes, observed species richness is a poor underestimate of true richness. The jackknife estimator and Chao's estimator both improve the estimate of species richness, but they are imprecise and can seriously overshoot the true richness value when the community includes many rare species. The bootstrap estimator. on the other hand, gives a better estimate than observed richness. Bootstrap estimates are also less variable and less likely to overestimate true richness, independently of how frequent rare species are in the community. This estimator provides a better, but conservative, estimate of true richness than observed richness and should be used to correct for inadequate host sampling. Data from natural communities suggest that the use of richness estimators is often justified, and that many parasite species may regularly escape detection.