The relation of host density to reproduction of the chalcid parasite Dahlbominus fuscipennis (Zett.) was examined in a previous study (Burnett, 1954). When the parasite searched in a naturally varying physical environment for hosts–cocoons of sawflies that attack conifers–distributed over plots on a grass lawn, the influence of host density on parasitism was similar to that obtained in laboratory experiments. As the irregularities of the meteorological factors did not appreciably alter the relation of parasitism to host density it was of interest to determine whether the additional discontinuities of a natural environment would.