0 Reiczigel, Jen 1998 Journal of Parasitology 84 3 491-498 38777 ecology lice phthiraptera specificity communities arthropods parasitism impala model Coexisting ectoparasite species tend to evolve different site-specificities due to site-segregation. This may result either from a direct interaction between ectoparasite species such as competition for resources or from the selective pressure upon parasite populations exerted by host defence, i.e. a host-mediated interaction. Here we present an individual-based simulation model for the coevolution of one host and two parasite species to test this latter hypothesis. Parasite species are generalists at the start of the simulation process and develop site-specificities due to selective pressure exerted by the hosts. Hosts are selected to maximise the effectiveness of defence. We describe certain conditions that affect the emergence of site-segregation under the following assumptions: (1) parasite populations are not subject to resource limitations but are limited directly by host defence as pdfJournal http://www.phthiraptera.org/Publications/3170.pdf