This study represents the first of its kind to evaluate the importance of spatial and temporal co= incidence between parasitic insects and their insect hosts. Extensive and intensive field observations here demonstrate that lack of coincidence is possible between the European pine sawfly, Neodiprion sertifer Geoff. and its three most important parasites. In the ichneumonids, Exenterus canadensis Prov., spatial coincidence is nearly perfect, while temporal coincidence may be poor. The bombyliid, Villa sinuosa (Wd.) shows imperfect spatial coincidence, but good temporal coincidence. In the ichneumonid Pleolophus basizonus (Grav.) both spatial and temporal coincidence may be imperfect. The degress of coincidence are related to the physiology of development and the behaviour of the parasites and their host in their variable environmental contexts. The interrelations of host density, parasite density, time of contacts, number of eggs laid, and number of hosts attacked, were studied experimentally with P. basizonus. The results were more closely described by the predictions of the Holling than of the Watt equation. The distribution of eggs in hosts was more often adequately described by the negative binomial than the Poisson equation for probabilities. The effect of asynchrony in the species studied cannot be considered without reference to superparasitisms. At low host densities superparasitism largely buffers effects of decreased synchrony, but its buffering diminishes to an asymptote as host density increases. In any given generation, imperfect spatial coincidence has little effect where attack is asymptotic. Simulations of host-parasite interactions over 25 to 35 host generations showed that for P. basizonus and E. Canadensis, host and parasite populations would become stable after passing through one or more oscillations even when, temporal and spatial coincidence are reduced. Predictions for V. sinuosa indicate its inability to restrain host populations even at perfect coincidence. / Science, Faculty of / Zoology, Department of / Graduate