Seasonal occurrence of fruit flies was studied by male trap captures from 1992 to 1994 in three natural areas invaded by strawberry guava (Psidium cattleianum) located at elevations of 100 m, 480 m and 720 m on the windward wet coast of Réunion Island (Indian Ocean, French overseas). Strawberry guava fruit infestation was monitored during harvest. The main fruit fly species captured on all sites was Ceratitis rosa (Karsch). C. capitata (Wiedemann) and C. catoirii (Guérin- Mèneville) were occasionally trapped at elevations of 100 m and 480 m. The study of fruit flies emerged from pupae collected from fruit showed that strawberry guava is an host for these three species from sea level to an elevation of 500 m. Only C. rosa infested fruit at higher elevations. As for the captures, this species was by far the most important in infested fruit. C. rosa populations were low all year round except during the strawberry guava harvesting period : populations increased rapidly when fruit reached maturity and decreased soon after harvest. Trap catches during harvest had no clear relation with the elevation : they were always more important at 100 m and 720 m than at 480 m. Whatever the elevation, trap catches were more important during the 1993 harvest season than during the 1994 one. The percentage of infested fruit increased more rapidly at lower elevations than at higher elevations and could reach very high levels (95% of infested fruit); it was not correlated with fruit quality (soluble solids, pH) at harvest. These results provide new information on the population dynamics of the Natal fruit fly and are useful to define spraying programs against this pest in strawberry guava orchards.