A pheromone-based, mass-trapping system in a commercial plantation of 20-yr-old oil palm,Elaeis guineensisJacq., in Costa Rica, was used to assess changes in abundance and distribution ofRhynchophorus palmarum(L.) and the incidence of red ring disease. The spatial distribution of trapped adult weevils was analyzed using Taylor power law and Iwao regression technique from 243 traps over a 30-ha area, that were sampled at 1-mo intervals from September 1991 to January 1993. Both methods of regression provided a good fit of the data. Indices of aggregation indicated that the adult population was initially highly aggregated, but tended to become increasingly random as trap catch declined over time. Three-dimensional graphical depiction of trap counts from October to January over a 1-yr period illustrated that aggregation was concentrated at the periphery of the study site, suggesting high captures of immigrants. At the end of 17 mo of continuous trapping over the census zone,>62,500 weevils (≈94 per ha per mo) had been removed. Mean trap counts declined from 32.4 weevils per trap per month in the first 70 mo of trapping to 6.4 per trap per mo in the later 9 mo of trapping. Because trap catches remained uniformly low in the later interval and there was no indication of an expected rise in populations during the dry season (December-March) encompassed in this interval, it is likely that the mass-trapping effect was responsible [or maintaining the population at low density. After 5 mo of continuous trapping, monthly incidence of red ring disease in areas that trapped<20 weevils per trap per month declined to2 within the study site.