The population fluctuation of the melon fly, Dacus cucurbitae was investigated in northeastern Taiwan. The study was conducted from both along the coastal plain of Pacific Ocean and nearby inland areas of I-Lan County from June, 1989 to the end of September, 1991. The highest population density of the melon fly was observed in July and August; it then decreased in September and reached its lowest level from December to May. Higher temperatures, a longer duration of sunshine, and increased plantation activity were the most important factors influencing the population density of the melon fly in these areas.