Behaviour and activity levels were measured in varying lemmings experimentally infected with the heteroxenous parasite, Sarcocystis rauschorum to test the hypothesis that the parasite alters behaviour of this intermediate host and thereby increases probability of transmission to the definitive host, the snowy owl (Nyctea scandiaca). Measures of short-term activity levels on a running wheel indicated no effect of the parasite, either directly, or indirectly as a result of illness. We observed behaviour of infected lemmings placed in an "open field" (arena). Lemmings would increase their susceptibility to predators if they spent more time away from cover, used crypsis (stationary postures) less, spent more time exploring (especially in unfamiliar areas), or responded inappropriately to threats from predators. We found that only exploratory activity showed significant change after infection. The frequency of exploratory activity increased and became disassociated from the usual fear response. This may increase the lemmings' susceptibility to aerial predation. The mechanism for this effect is unknown, but neurological lesions have been observed. The examination of the modes of transmission of the S. rauschorum parasite within lemming populations and of a possible fecundity compensation strategy adopted by the lemmings, and their relevance to population control, are suggested as areas for future study.