Abstract: The spatial distribution of medfly Ceratitis capitata (Diptera: Tephritidae) adults captured in the traps was analyzed in citrus orchards where mass trapping was applied to control the pest, with the objective of understanding how the method works, to determine the factors which influence it and to suggest changes which could improve its efficacy. A network of traps for mass trapping was established in 31 citrus orchards In Ibiza Island, from 2006 to 2008. The traps were of the tephri-trap type and baited with Tripack as attractant; they were uniformly distributed at a density of 50 per hectare following the standard recommendations. All traps of the mass trapping arrangement were sampled at intervals of 30 to 45 days by counting all flies captured on each trap. In all, 110 samplings, with a mean of 102 traps per sampling, were observed. Our results show that the establishment of the network of traps causes a barrier effect, with a gradual and rather constant reduction in medfly captures from the periphery to the interior of the area covered. The reduction rate observed is usually low: the average captures were 3.5 flies per trap and day (ftd) on the external edge row of traps and 2.3 ftd on the second row of traps, reaching 0.7 ftd on the sixth row located 70 meters inside the orchard. The following factors were analyzed for the barrier effect on medfly populations: size and shape of the orchard, presence of mature fruits, species and/or variety of fruit, month of the year, time elapsed since the mass trapping was laid out, and overall medfly abundance in the periphery of the area protected. The factors that showed significant influence were the size and shape of the orchard, and the month of the year: larger plots and plots with minimum ratio perimeter/area showed higher capacity in reduction of captures from the exterior to the interior of the plot, and the efficiency of the system of control increases in June and July. According to these results, the success of mass trapping can be measured by the capacity to get high levels of reduction between the external and the internal traps in the plot. The mass trapping technique must be applied in large areas, of several tens of hectares, in surfaces with a compact shape in order to minimize the perimeter, and increasing the density of traps in the periphery of the protected area. Traps should be installed when fruit ripening approaches but not before as no improvement was observed when traps were established in earlier periods of fruit development.