The phenology and population dynamics of the Mediterranean fruit fly, Ceratitis capitata (Wiedemann), were studied from 1991 to 1995 using Jackson traps baited with trimedlure and periodic fruit sampling in two orchards in Thessaloniki northern Greece (40.3° north latitude, 22.5° longitude). This area is located within the northernmost zone of establishment of the fly in Europe. No adults were captured from December to the end of June. The first captures were recorded from the end of June to August, depending on the year and orchard, and captures rates peaked in the autumn of each year. Significant differences were observed in adult population density and in the initiation of fly activity between two neighboring orchards (≈500 m apart) that differed in host fruit abundance and availability. The results of fruit sampling showed that apricots were the first fruits infested every year in the area of Thessaloniki. Though infested at low rates, they were very important for breeding the first summer generation, and also for the increase in C. capitata population later in the summer. Peaches and figs were important hosts for breeding the late summer and early autumn generations. Apples and other pome fruits were important hosts later in autumn (October–November), and also served as overwintering refuges for the larvae. Among 17 fruit species collected in the area 11 were found infested. Pecan [ Carya illinoensis (Wang) k. Koch] and Malus floribunda Sieb. (an ornamental tree) are reported here for the first time as hosts of C. capitata. Our findings suggest that C. capitata completes four to five generations per year in the area of Thessaloniki. Practical and theoretical implications concerning the population dynamics and the control of this fly are discussed.