We report here the first discovery of microvertebrates from the Mae Moh coal mine, Lamphang Province, northern Thailand. This discovery sheds new light on the age and paleoenvironments of the Miocene basins of northern Thailand. Previous investigations of paleomagnetic stratigraphy in the Mah Moh Basin demonstrated that the fossiliferous Q and K coal seams there date between 13.12–13.3 Ma. The microvertebrate fauna from the Q and K coal seams includes Tarsius sp., erinaceid insectivores and rodents, the last being represented by the monospecific genera Prokanisamys and Neocometes. These microvertebrate fossils are extremely similar to elements of the Mae Long fauna from the Li Basin, which had been estimated to date between 16–18 Ma. Accordingly, the Mae Long fauna in the Li Basin appears to be contemporaneous with the fauna from the Q and K coal seams in the Mae Moh Basin, suggesting that previous estimates of the age of the Mae Long fauna were 3–5 Ma too old. The discovery of numerous fragments of the primitive deer Stephanocemas cf. rucha and the pig Conohyus thailandicus in the Mae Moh Basin confirms the correlation suggested by the microvertebrates. As a result, there is no longer any reason to advocate a west-east chronological gradient for the opening of the intermontane basins of northern Thailand. Alternatively, dating of other Tertiary basins in Thailand suggests a south-north gradient. The Krabi Basin, located in peninsular Thailand, is of latest Eocene age, the Nong Ya Plong Basin in central Thailand dates to the late Oligocene, and basins in northern Thailand are middle Miocene in age. Analysis of mammalian faunas and pollen indicate that the middle Miocene was a period of dramatic climatic fluctuations in northern Thailand. The Q and K coal seams in the Mae Moh Basin show a temperate-tropical vegetation, whereas the uppermost coal layers testify to warmer tropical vegetation. Based on magnetostratigraphic data, these climatic fluctuations were synchronous with those observed in Antarctic deep waters. Climatic fluctuations during the middle Miocene cannot be used for purposes of correlation, because they appear to have occurred in an iterative fashion.