Hypotheses for the cause of population cycles have focused on the role of intrinsic mechanisms such as spacing behaviour and extrinsic mechanisms such as parasitism and predation. This paper examines the interaction between the two dominant hypotheses in the cycles of red grouse, parasitism and spacing behaviour. The influence of the caecal nematode, Trichostrongylus tenuis, on male aggressiveness was investigated using two approaches. First, the territorial behaviour of a group of male grouse experimentally treated with an anthelmintic to reduce parasite intensities was compared with a control group with natural intensities of infection. Second, the response of treated and control males to a novel conspecific territorial intruder were recorded using playback tests. Treated males, with reduced levels of parasitism, won significantly more territorial contests than control males and produced more aggressive behaviour in response to the playback recordings. These results show that parasite removal enhanced aggressive behaviour. Implications of these results on red grouse population dynamics are discussed.