A long-term study of trap catches and fruit sampling showed definitive habitat-related patterns of occurrence of populations of the Mediterranean fruit fly (medfly),Ceratitis capitata(Wiedemann), in the Hawaiian valleys of Makaha and Waianae. In general, seasonal and annual fluctuations in patterns of occurrence were influenced by seasonal and annual differences in abundance of coffee berries and distribution of other fruits. Three distinct areas of activity were observed. The most ecologically significant occurred in coffee in the upper part of Makaha and Waianae valleys with a mean catch of 0.78 and 0.32 flies, respectively, per trap per day. The other, dependent upon dispersing flies, occurred in the common residential area in the lower part of each valley, with a mean catch of 0.12 flies per trap per day in Makaha and 0.06 in Waianae. The most important host fruits were coffee, false kamani, common guava, mango, and mock orange. The stabilizing influence of coffee on the medfly population and the fly's propensity to disperse are discussed. Although several small populations exist in Hawaii, previous investigators have studied only the large populations, especially those in Kona, and in Kula, Maui. This is the first ecological study of a small population.