Dispersal, which is defined as any movement of individuals or propagules affecting gene flow across

space (Ronce 2007), is one of ubiquitous traits for living organisms. We have already known that there are

some kinds of selective pressures driving evolution of dispersal; for example, competition among sibs,

inbreeding avoidance, spatiotemporal resource availability, kin-cooperation, and so on. They are well-discussed in both theoretical and empirical studies, but other factors are almost underestimated

or neglected. Here I focus on parasite load. Parasite load is also ubiquitous and is considered to have negative impacts on host species, but as far as we know there are few empirical works discussing the

role of ectoparasite in host’s dispersal strategy (e.g. Sorci, et al. 1994) . In addition, almost all

theoretical works concentrate on the evolution of parasite virulence as local adaptation to host (e.g.

Frank 1996, Gandon 2002). Here I introduce simple mathematical models based on Hamilton and May

1977 (Nature) incorporating ectoparasite’s negative impact and horizontal transmission, and discuss

evolutionary dynamics in terms of evolutionary stability (ES; Maynard Smith 1982). By mathematical

analysis and numerical simulation, we got some results, which appear to be biologically strange. These

results suggest the importance of ectoparasite in host dispersal.