Metepa-sterilized males ofDacus oleae(Gmelin) mixed at different ratios with normal males were allowed to compete for normal females. Sexual competitiveness of sterile males was compared with that of normal males based on mating activity and egg hatch. The effect of competition stress on the mortality of insects also was examined.Mortality of sterile males was lower than that of normal males when competition stress was absent. Mortality of sterile males competing with normal males at 1:1 and 2:1 ratios respectively, is higher than that of normal males. At the 4:1 ratio sterile males show the same low mortality as irradiated males caged alone. Mortality of normal males was not affected by competition stress. Mortality of normal females which were introduced into the male cages every week for mating was the same for all experiments during the first two weeks. During the following weeks mortality was higher when females mated with normal males or when they were outnumbered by males by a factor of at least 5.Mating competitiveness of sterile males was the same as that of normal males. On average, more than 72% of total matings of both sterile and normal males caged separately occurred during the first day of their contact with females. The percentage of sterile males caged alone which mated was lower than that of normal males caged alone during the first week and higher during the third week.Competitiveness of sterile males based on egg hatch was the same as that of normal males in all cases except during the first week and only when they occurred in a 1:1 ratio.