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Captures of Dicrostonyx groenlandicus on northeastern Devon Island peaked in 1969 and 1973, were higher than expected in 1971, and were minimal (< 5 per 1000 trap-nights) in 1970 and 1972. Captures declined from July to August in at least five of seven summers and may have declined in a 6th year (1973). No change was detected in 1972. Body size was maximal in 1973 for mature and immature males and females and for all pelage (=age?) classes. Sexual maturity was delayed, at least in males, in 1973. Wintering females conceive under the snow in May and the litter is weaned at the normal time of snowmelt. Two other summer cohorts are produced by surviving old females and early maturing females of the first litters. None of the late summer cohort matures before the onset of winter. No female examined bore more than two summer litters. Mean summer litter size was 5.7 with no significant variation between years. Mean sex ratio was 50.3% male. It is suggested that density-related effects ought to occur in winter, rather than summer, but evidence on this point is equivocal. Arctic adaptations of D. groenlandicus are discussed.

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