The differences in biological characteristics between two local pine needle gall midge populations in Seoul and Muan (Jeollanam-Do), where this insect was first reported from Korea, were studied with special reference to the effect of temperature on their development. 1. The threshold temperatures for overwintered larval development in Seoul and Muan populations were 5.6$^{circ}C$ and 6.8$^{circ}C$ respectively; the accumulated effective day-degrees for the development of those larvae were 900D$^{circ}$ for the Seoul population and 1020D$^{circ}$ for the Muan population. 2 There was no difference in the threshold temperatures for pupal development between two populations; they were generally higher than those for the larval development. 3. The overall threshold temperatures calculated for the complete develpment from overwintering larvae to adult emergence were 6.3$^{circ}C$ for Seoul population and 8.9$^{circ}C$ for Muan population, and the accumulated effective day-degrees for adult emergence were 859 for the Seoul population and 881 for the Muan population. 4. The adults of the Muan population emerged 16-43 days later than those of the Seoul population when they were kept at the same forest site in Seoul. 5. From these results, it is concluded that there is a significant difference between Seoul and Muan populations in their developmental biology, indicating the differentiation of ecological races.