Late Tertiary and early Quaternary fossil beds in the arctic regions of North America have yielded abundant, well-preserved remains of plants and arthropods, documenting the existence of coniferous forests in the high arctic latitudes. Nearly all of the beetle (Coleoptera) specimens in these fossil assemblages represent extant species. We have applied the mutual climatic range (MCR) method of paleotemperature analysis to fossil beetle assemblages from 11 sites to estimate mean summer (Tmax) and winter (Tmin) temperatures. We found that arctic Tmin values during the latest Miocene and Pliocene were substantially warmer than they are today. The MCR estimates therefore support the scenario derived from the paleobotanical data, namely that arctic Pliocene climates were far less continental. Several Pliocene-age assemblages from the high Arctic yielded Tmax estimates 9–10°C warmer than modern values at the sites. This is the same degree of warming required to allow coniferous forests to grow in the high Arctic. By 3 million years BP, a cooling trend is marked in the paleobotanical and fossil beetle evidence from Alaska. All assemblages dating between 5.7 and 2 million years BP yielded calibrated Tmax values between 12.4 and 13.8°C, regardless of location. Thus the insect fossil data support the theory that there was far less latitudinal gradation in temperatures during the Late Pliocene than there is today. Our reconstructions suggest regional climatic cooling (especially winter temperatures) began by at least 2 million years BP.