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The importance to the biotic community of various species of lemmings in arctic and subarctic regions has long been recognized, but there is little known about the ecology of these mammals. of the two species that occur on the Arctic Slope of Alaska, namely, the collared lemming, Dicrostonyx groenlandicus rubricatus (Richardson), and the brown lemming, Lemmus trimucronatus alascensis Merriam, during the spring of 1949 the writer had the good fortune to observe a cyclic decline in the population of the brown lemming on the Arctic Coast of Alaska. Observations were made during the peak density preceding this decline and were continued for more than a year subsequent to it. It is the purpose of this paper to present the results of these studies.