Bactrocera invadens Drew, Tsuruta & White (Diptera: Tephritidae) invaded Kenya in 2003. Before the arrival of B. invadens, the indigenous fruit fly species Ceratitis cosyra (Walker) was the predominant fruit fly pest of mango (Mangifera indica L.). Within 4 yr of invasion, B. invadens has displaced C. cosyra and has become the predominant fruit fly pest of mango, constituting 98 and 88% of the total population in traps and mango fruit at Nguruman, respectively. We tested two possible mechanisms responsible for the displacement namely; resource competition by larvae within mango fruit and aggression between adult flies. Under interspecific competition, larval duration in B. invadens was significantly shorter (6.2 ± 0.6-7.3 ± 0.3 d) compared with C. cosyra (8.0 ± 1.2-9.4 ± 0.4 d). Pupal mass in C. cosyra was affected by competition and was significantly reduced (7.4 ± 0.3-9.6 ± 0.6 mg) under competitive interaction compared with the controls (12.1 ± 1.5-12.8 ± 1.1 mg). Interspecific competition also had a significant adverse effect on C. cosyra eclosion, with fewer adults emerging under co-infestation compared with the controls. Interference competition through aggressive behavior showed that fewer C. cosyra (3.1 ± 0.8) landed on mango dome compared with the controls (14.2 ± 1.5) when adults were mixed with B. invadens adults in Plexiglas cages. Similarly the number of times C. cosyra was observed ovipositing was significantly lower (0.2 ± 0.2) under competitive interaction compared with the controls (6.1 ± 1.8). Aggressive encounters in the form of lunging/head-butting and chasing off other species from the mango dome was higher for B. invadens compared with C. cosyra. Our results suggest that exploitative competition through larval scrambling for resources and interference competition through aggressive behaviors of the invader are important mechanisms contributing to the displacement of C. cosyra by B. invadens in mango agroecosystems.