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The damselfly, Nehalennia irene (Hagen), has two distinct female colour morphs. Individuals of one morph have male-like colouration and pattern (androchromes), whereas gynochromes are different from males and androchromes in these respects. In several damselflies, such female-limited polychromatism is attributable to a single genetic locus. We developed six polymorphic genetic markers, which were codominant, to test for genetic differentiation in N. irene, collected from two sites located 8 km from one another in eastern Ontario, Canada. Based on three censuses spanning a 10 year period (1992–2001), morph ratios differed consistently and significantly between these two sites. However, subpopulations at these sites were not genetically differentiated with respect to the putatively neutral markers. Our results suggest that site differences in morph ratios of female N. irene cannot be explained by genetic drift, but are consistent with spatially variable selection operating on different morphs, perhaps mediated by male density. Alternatively, morph type may be a plastic trait and cues for induction may differ between sites. [ABSTRACT FROM AUTHOR]