We develop a formal theory, based on simple population dynamics constructs, to identify how proportional refuges to parasitism may act to determine the number of parasitoid species sampled per host species. A series of mathematical population models based on biologically realistic phenomena predict one of two basic species richness patterns. If the host population is sufficiently exploited, then a domed relationship results between refuge and richness; otherwise, depending on the types of parasitoids present and the initial population levels, the basic pattern is either domed or simply monotonically decreasing. We suggest that simple rules, centering on proportional refuges to parasitism and the finite rate of increase of the host population, explain the richness patterns present in real parasitoid assemblages.