The ecological behaviour ofCochliobolus miyabeanus(Ito et Kurib.) Drechsl. ex Dast.(Bipolaris oryzae(Breda de Haan) Shoem.), causal agent of brown spot of rice(Oryza sativaL.), was investigated in a tidal mangrove swamp at Rokupr Experimental Farm, northwestern Sierra Leone. Disease increased where transplanting was delayed. This was probably due to the coincidence of flowering with favourable environmental conditions for disease development from November to February. Nitrogen fertilization at 40kg N ha−1significantly (P= 0.05) reducedC. miyabeanusincidence in 1983, but not in 1984 and 1985 cropping seasons at the same site. Grain yields of ROK 5, a medium‐duration improved rice cultivar(ca.150 days) were significantly reduced in late‐transplanted rice crops (September to November) in spite of adequate N fertilization. Mangrove mud was not an important source ofC. miyabeanuspropagules. Incidence of leaf scald caused byMonographella albescens(Thum) Parkinson, Sivanesan and Booth(Microdochium oryzae(Hashioka and Yokogi) Samuels and Hallet) and leaf smut caused byEntyloma oryzaeMiyake were generally stimulated by N application.