Research was carried out on 24 orange plantations in Costa Rica in order to determine the sensitivity to benomyl of Mycosphaerella citri Whiteside - fungi causing the citrus greasy spot. Only thirteen samples yielded enough ascospores to determine their sensitivity to benomyl. Twelve of the samples represented an area of approximately 4.000 ha from the Northern part of Costa Rica (the cantons of San Carlos and Los Chiles), and one was from a nonsprayed home orchard in Orosi, in the Central Valley located more than 250 km away from San Carlos. AIl of the samples from the Northem region yielded benomyl resistant ascospores, although benomyl is not normally used in those areas. Conversely, 99% of the ascospores obtained in Orosi were sensitive; this suggests that the M. citri wild population is benomyl-sensitive. It was found that 75% of the orchards sampled came from the same nursery, where the phytosanitary program includes five benomyl applications ayear. Furthermore, 10 M.citri samples obtained from this nursery were benomyl-resistant. This suggests that the benomyl-resistant isolations found in the plantations were introduced there through the nursery stock.