Studies of population fluctuations of the oriental fruit fly,Dacus dorsalisHendel, were conducted in two guava orchards from January 1980 to December 1981 using methyl eugenol traps. Three distinct population peaks were recorded in March/April, May/June and September/October during both years. These peaks coincided with the ripening of guava and mango fruits which are the major hosts ofD. dorsalis.Abiotic factors also played an important role in regulating the fly population. Trap catches were significantly and positively correlated with maximum and minimum temperatures, day degrees (or thermal units) and maximum relative humidity. Trap catches were significantly and negatively correlated with minimum relative humidity. The placement of fruit fly traps in different directions and positions in orchards had no apparent effect on the catches of flies. However, traps placed in the edges of the orchards caught significantly higher numbers of flies than traps placed in the centre.