Technical Abstract: Size and nutritional status, two factors that can determine male mating competitiveness, are usually studied independently. Here we investigated the interactions between size and adult diet on the sexual competitiveness and female fitness of two tephritid flies with contrasting natural histories, the Mexican fruit fly, Anastrepha ludens and the guava fruit fly, A. striata. Small, medium and large males were provided with either a low quality (sucrose offered every third day) or high quality diet (sucrose and protein offered ad libitum). Males of both species fed on a high quality diet copulated significantly more often than males fed on a low quality diet. For A. ludens, large size and high quality diet were important in territory defence by resident males, however, only size affected the likelihood of success by invading males. In contrast, for A. striata, diet was only important in territory defence by invading males. For A. ludens females, fertility was affected by male size, while maximum longevity was significantly shorter when females copulated with a low quality fed male regardless of size. In A. striata, females discriminated strongly against low quality diet fed males but not against smaller males. However, females that copulated with medium and small males exhibited lower fecundities than those copulating with large males. Females may be manipulated into copulating with smaller males if there is sexual conflict over mating decisions. We discuss these results in view of the costs for females of mating with smaller males and their inability to distinguish between males of different conditions, as well as interspecific differences in mate choice and postcopulatory consequences for females.