Eudonia mawsoni, the only moth breeding on subantarctic Macquarie Island, was sampled to measure density and sex ratios in different habitats and at different altitudes, and to investigate the distribution of colour morphs. Sites sampled ranged in altitude from 20 to 433 m above sea level, and included mire, short grassland, herbfield, feldmark and moss. Adult moths were widespread, occurring from sea level to the highest mountain (433 m a.s.l.). Mean density ranged from 0 to 2.3 m−2. Frequencies of males and females differed significantly for only 6 of the 24 sampling events, with males predominant at 5 of those 6. Females predominated only in collections from a pure moss site, where they appeared to gather to oviposit. In some vegetation types, moth sex proportions differed significantlly from average proportions, with the proportion of females elevated in feldmark and moss, and depressed in mire. Colour morph distribution was significantly affected by both sex and altitude, with females darker than males and both sexes with a greater proportion of darker individuals at higher altitudes. Larvae occurred at sites ranging from sea level to 370 m a.s.l., usually in mosses. Their diet included mosses and some non-moss plant material.