The first early to middle Miocene terrestrial mammalian fauna from Thailand (the Nong Hen Local Fauna) was recovered at a depth between 887 and 894 meters in the Thai Shell Exploration and Production Company Ltd. Nong Hen-I(A) well located in the northern part of the country. More than 100 mammalian jaws and teeth were recovered. Most specimens were from a single new species of bat, Mormopterus ( Hydromops) nonghenensis but two other bats were also present as well as a small rodent and some other, larger mammal. The preponderance of bats in this assemblage is concordant with its having been recovered from a karst deposit. The stage of evolution of M. ( H.) nonghenensis suggests that the Nong Hen Local Fauna is late early to middle Miocene in age.