Abstract:Researchers have questioned whether variation in population density influences the maintenance of two or more morphs within and between insect populations. Two distinct morphs of femaleNehalennia irene(Hagen) (Odonata: Coenagrionidae) occurred at each of eight sites in eastern Ontario, Canada; one morph (i.e.andromorph) was coloured and patterned like the conspecific male. We found significant differences in relative frequencies of andromorphs and in male density between sites; frequency of andromorphs ranged from 2–28% and was positively correlated with male density. Sites with high densities of males were located closer to low-density sites than to other high-density sites, indicating that high-density sites were somewhat independent of one another. We also found spatial variation in density of males within sites; males were present almost exclusively at edges of ponds or marshes. Proportionally more andromorphs than heteromorphs were found at the edges of sites, where male-female interactions were likely frequent. Sex ratios at the edges of sites were most strongly male-biased at two of three high-density sites, indicating that males may force non-receptive females away from the edge of high-density sites. Spatial and temporal variation in male density ofN. irenemay be an important factor influencing morph coexistence.