Female-limited colour polymorphism occurs in many damselfly species, where one morph resembles the male (andromorph) and the other is dissimilar (gynomorph). Explanations for this phenomenon vary, but most assume that andromorphism has arisen in odonates, as a response to excessive male harassment. Here, we quantify the extent of continental and seasonal variation in female morph frequencies in a widely-distributed damselfly and ask whether the spatiotemporal patterns in andromorph frequency can be understood on the basis of sexual harassment theory. We sampled the damselfly, Nehalennia irene (Hagen) among regions across Canada, and at several sites, over the reproductive season, within Central Canada. Andromorph frequencies ranged from 0 to > 90% across Canada. In particular, sites in Western Canada had consistently high andromorph frequencies, whereas andromorph frequencies among Central sites were lower and variable and, among Eastern sites, were lower still (except one site) and relatively invariant. For populations in Central Canada, both andromorph frequencies and population densities varied significantly over time, reaching a peak mid-season. As expected, morph frequency covaried significantly with estimates of male harassment in some cases, but estimates of male harassment did not consistently account for variation in morph frequencies within all regions. Additional factors such as genetic drift may influence morph frequency at the edge of a species’ range. Future work also should test, and attempt to explain causation, for seasonal variation in morph frequency. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society, 2007, 90, 501–508.