Bactrocera invadens Drew, Tsuruta & White (Diptera: Tephritidae), an invasive fruit fly species of Asian origin, was detected in Kenya in 2003, and is now well established in several parts of the country. We assessed the host range of this major quarantine pest in Kenya by collecting a wide range of cultivated and wild host plants from December 2004 to April 2006. Fruit were collected from 90 plant species representing 40 families from the Coast, Eastern, and Rift Valley provinces of the country where the fly population had been observed to occur in large numbers and where fruit and vegetable production is predominant. Fourteen plant species, among them cultivated and wild fruiting species, were found to be hosts of B. invadens. Fruit of mango, Mangifera indica L. (Anacardiaceae); banana Musa sp. AAA (Musaceae); and citrus [Citrus limon (L.) Burm.f. (lemon), Citrus reticulata Blanco (tangerine), and Citrus sinensis (sweet orange) (all Rutaceae)], were among the cultivated species heavily infested by B. invadens. Marula Sclerocarya birrea (A.Rich) Hochst. (Anacardiaceae) and Terminalia catappa L. (Combretaceae) were found to be the most infested noncultivated plants. These wild plants evidently ensure that sufficient reproductive bases exist for B. invadens during the off-season when the cultivated hosts are not in fruiting. In laboratory host preference studies, mango and banana were found to be the most preferred host plants among the nine cultivated plant species tested.