The bamboo borer, Omphisa fuscidentalis, is a moth found in northern Thailand, Lao and Myanmar and its larvae feed on the inner pulp of bamboo shoots. In a tropical highland (about 500 m sea level) forest at 19°N near Chiang Mai, Thailand, the larvae feed on at least 5 bamboo species. Nucleotide sequence analysis of the region of mitochondrial cytochrome C oxidase subunit 1 gene amplified by the polymerase chain reaction (PCR) verified that larvae collected from different bamboos belong to the same species. Adults appeared in early August and laid clusters of eggs on the newly grown bamboo shoot. The newly hatched larvae bore a hole in the shoot, enter an internode of the shoot and feed on the inner pulp. After maturation in September, the larvae remain in an internodal cavity of bamboo for up to 9 months, from September to the following June. Number of larval instars was estimated by measuring the width of head capsules remained in internodes of bamboo shoots. The growth curve of the width fitted to Dyar's law and the mature larvae were estimated to be 5th instar. Mature larvae were collected in the field each month and their body weight, head capsule width, protein and fat contents and hemolymph ecdysteroid titer were measured. Body weight continuously decreased during the 9 months whereas head capsule width remained constant. Fat content fluctuated during this period while protein level remained at a similar level until March, after which it significantly increased. During this period, hemolymph ecdysteroid concentrations remained low. Current results show that the bamboo borer larvae enter diapause at the end of feeding period of the fifth (last) larval instar and the larval diapause lasts until June.