The effect of Schistocephalus solidus infection on reproductive development and behaviour of male three-spined stickleback Gasterosteus aculeatus from an upland lake (Llyn Frongoch, Wales, U.K.) was investigated. Uninfected and infected males were collected on two separate occasions during the 2005 breeding season and encouraged to build nests under favourable laboratory conditions. Male nuptial colouration, courtship levels and nesting activity were recorded daily over two separate 21 day studies. Completed nests were removed, encouraging males to build multiple nests. On termination of each study, males were dissected and the kidney and any S. solidus plerocercoids were removed and weighed. In contrast to uninfected males, which readily built multiple nests, no infected males completed a nest during either study. Infected males also exhibited significantly reduced courtship levels, nesting activity, nuptial colouration, kidney development and body condition, compared with uninfected fish. Among infected three-spined stickleback, courtship and nesting behaviours tended to be more severely impacted in fish harbouring heavier plerocercoid burdens. In contrast to other populations, male three-spined stickleback from Llyn Frongoch appear to be unable to overcome the negative impacts of infections on reproductive development and behaviour when provided with short-term favourable conditions. Discrepancies between populations might have been related to the trophic status at the site of capture, emphasizing the importance of studying the biological effects of infections under different environmental conditions.