The reproductive success of a foraging parasitoid may be limited by the number of eggs that she produces and/or the number of hosts that she can locate. Despite the significance for population dynamics and numerous areas of behavioural ecology, the relative importance of these factors remains an issue of contention. Attempts to resolve this controversy have been hindered because estimating the importance of factors limiting reproduction in the field can be extremely laborious and time consuming. We show how sex ratio data can be used as a relatively easy method to indirectly estimate the relative importance of the factors limiting reproduction. Sex ratio data from 48 samples of eight species suggest that: (a) the extent of host or egg limitation in a species varies between site collected and time of year; and (b) on average, species are at an intermediate position on the egg/host limitation continuum, with a bias towards host limitation.