A simple new measure of parasite aggregation is described, the index of discrepancy (D). It quantifies the difference between the observed parasite distribution, and the hypothetical distribution that corresponds to the ideal case where all hosts harbour the same number of parasites. This index, computed for parasite distributions obtained from the literature, is compared to 2 other measures of aggregation, the variance to mean ratio and the parameter k of the negative binomial distribution. Both k and D indicate that aggregation decreases when the prevalence of infection and the mean number of parasites per host increase, while the variance to mean ratio suggests the opposite. Since an increase in prevalence means that parasites exploit a greater proportion of the available hosts and are thus not concentrating in only a few, aggregation should be inversely proportional to prevalence. Unlike k and D, the variance to mean ratio is a host-centered measure that is not very sensitive to the distribution of parasites. The index of discrepancy, on the other hand, is not only much easier to compute than k, but focuses on the difference between an ideal, uniform distribution and the one actually displayed by parasites. Since what it measures is what parasitologists mean by aggregation, the new index appears to be a more adequate measure of aggregation than other measures currently used.