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The rice brown spot disease caused by Cochliobolus miyabeanus (Ito et Kurib.) Drechsl. ex Dast. = (Helminthosporium oryzae Breda da Haan) is a key disease in mangrove swamp rice ecosystem in Sierra Leone. On average, grain yield losses of 8.2–23.0% were caused by the pathogen in mangrove swamps of northwestern Sierra Leone in the 1983, 1984 and 1985 cropping seasons. Disease attack was closely linked with adverse soil conditions and favourable meteorological factors of long dew periods and moderate temperatures. However, attack by C. miyabeanus at the reproductive growth stage of rice caused the most damage because of direct effect on the grains.