Host preference is strongly influenced by factors such habitat and environment, biochemical characteristics of the hosts, and physiological and genetic characteristics of the fly. Ceratitis capitata is the dominant species in the area, with high densities present from late spring to late summer. Anastrepha fraterculus is present in low densities throughout the year. With favorable climatic conditions and susceptible hosts, the number of females per trap per day was 0.02 during the 1989–1993. Fruit samples were collected weekly from October, 1989, through November, 1993, from citrus orchards, backyard orchards, and native flora near Concordia, Entre Rios, to establish the hosts of C. capitata and A. fraterculus in this region. Twenty-four species were identified as hosts for C. capitata, most of which belong to the families Rosaceae, Solanaceae, and Rutaceae. Collected fruits were taken to the laboratory, counted and weighed, immersed in a 5% solution of sodium benzoate for five minutes, and placed in plastic containers with sterile humid sand.