An evaluation of the geographic distribution of approximately equals 60% of the 2809 described Nearctic Ichneumonidae shows that of the species whose latitudinal ranges contact a given latitudinal band, the number per million square kilometres is greatest between latitude 37.5 degree and 42.4 degree N, with approximately equals 900 species/10 super(6) km super(2). The most severe absolute drop in species richness occurs in moving southward from the 37.5 degree -39.9 degree band to the 35.0 degree -37.4 degree band, with virtually no change in land mass over the same latitudinal change, and the smallest drop in species richness occurs in moving from the 32.5 degree -34.9 degree band to the 30.0 degree -32.4 degree band, where the land mass is almost halved. Of species whose southern range ends in a particular latitudinal band, the number per million square kilometres is greatest for 32.5 degree -34.4 degree N and drops away gradually in both directions. These results are consistent with the hypothesis that parasitic Hymenoptera do not increase in species richness with decreasing latitude to the same degree as their hosts. Various hypotheses are discussed as to why this might occur in Ichneumonidae. none