| Summary: | Circadian rhythms of two allopatric species, Clethrionomys gapperi and C. rutilus were studied near their common border at Heart Lake, Northwest Territories, Canada, in winter and spring of 1965-1966. Circadian rhythms of C. gapperi were also studied at Edmonton, Alberta, in the winter and spring of 1964-1965. Activity was measured by recording passages through a door in cages exposed to normal meteorological conditions of the forest floor. Daily peaks of activity of each species were of longer duration and higher amplitude in spring than in winter. C. rutilus was polyphasic in winter, nocturnal in spring. C. gapperi at Heart Lake was nocturnal in winter but had peaks of activity persisting from dusk each evening to noon of the following day. C. gapperi at Edmonton was diurnal in all seasons.
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