Control costs and direct crop losses due to infestations by the spiny blackfly, Aleurocanthus spiniferus Quaintance, were studied in orchards of a 667 ha citrus estate, Inyoni Yami Irrigation Scheme at Tshaneni, Swaziland. In the 1995/96 season the control costs and direct losses at this estate amounted to R 476 000 (= US$ 86 500). This excludes the reduction in growth, the additional costs of sorting sooty mould-stained fruit and the potential reduction in crop production of the next season. To import, rear and establish the spiny blackfly parasitoid, Encarsia cf. smithi (Silvestri), including the project overheads, amounted to R 169 750 (= US$ 30 900). Because of the classical biological control that was achieved, no chemical control of this pest was necessary during the following two seasons and no further losses of fruit were incurred due to sooty mould. The amount spent to establish biological control of the spiny blackfly in parts of Southern Africa, even if compared to the direct losses and control costs of a single season on one citrus estate, was exceptionally cost effective. A benefit ratio was calculated at 2.8.