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A field trial was conducted to test the efficacy of an oil formulation of the entomopathogenic fungus, Metarhizium flavoviride Gams & Rozsypal (Deuteromycotina: Hyphomycetes), as a control agent for the desert locust, Schistocerca gregaria Forskal (Orthoptera: Acrididae), in Mauritania. The treatment was applied to hopper bands in the field. In caged samples of the treated insects, 99% mortality was observed in 15 days. On average during application only 51% of the hoppers received spray droplets directly, the additional mortality being due to pick-up of spores from the splay residue. This mortality was assessed by exposing insects to a treated plot at intervals, and monitoring disease levels during subsequent incubation in cages. The spray residue remained highly infective for a period of 6 days after application. The number of nymphs in the hopper bands was estimated using a computer-based photographic image-processing technique. Unfortunately, most of the hopper bands dispersed into small groups, but a population reduction could be demonstrated for one hopper band. Maximal daily mortality in the field was observed 10-11 days after application, compared with 6-10 days in cages.