The experiment was performed from September, 2011 until April, 2012 in order to determine the infestation stage of fruits, the population dynamics and the diversity of fruit files (Diptera: Tephritidae) in a guava orchard in the western region of São Paulo State, Brazil. The population dynamics of fruit flies were obtained from three McPhail plastic traps baited with hydrolyzed protein. To evaluate the infestation of fruit flies, 180 fruits were chosen in which fifteen of them were fortnightly bagged or unbagged. Fruit length and diameter were measured to define the fruit development stage most susceptible to natural fruit fly infestation. We captured a total of 300 Tephritidae specimens: 102 specimens of Anastrepha spp. (34.1%) and 198 of Ceratitis capitata (Wied.) (65.9%). Four species of Anastrepha fraterculus (Wied.), were recovered from McPhail traps: A. obliqua (Macquart), A. striata Schiner and A. sororcula Zucchi. A total of 31 adults were found in the fruits: 2 C. capitata (6.5%) and 29 Anastrepha (93.5%); A. obliqua and A. sororcula. Fruits with 2.6-cm of mean diameter were infested by fruit flies.