Neosilba perezi (Romero & Ruppel) is known as the cassava shoot fly or shoot fly, because, unlike other species of its genus, its larvae feed exclusively on shoots of cassava (Manihot esculenta Crantz). These larvae kill the affected cassava shoots. Natural parasitism by a braconid, Phaenocarpa neosilba Arouca & Penteado-Dias, 2006, on shoot fly larvae was studied at 3 locations in the southwestern region of the state of Sao Paulo, Brazil. Larvae-infested shoots of cassava were collected at 3 distinct locations where cassava crops were grown. The shoots were kept in plastic boxes for pupation and emergence of adult flies and the parasitoid. The percentage of parasitism was separately correlated with temperature, rainfall, and age of the cassava plants. The average percent parasitism was 24.36%. A negative correlation between temperature and percent parasitism was observed at 1 location. At all 3 study areas percent parasitism of N. perezi showed a negative correlation with plant age. Therefore, given the high percent parasitism observed at the 3 locations, natural biological control may be an important factor in the population dynamics of N. perezi.