Crop protection problems of smallholders farming maize in the Chokwe Irrigation Scheme in southern Mozambique were investigated in a long-term field study programme during 1984–1990. Downy mildew, Peronosclerospora sorghi (Weston & Uppal) C.G. Shaw, maize streak virus, stemborers (mainly Chilo partellus Swinhoe) and the multimammate rat (Praomys natalensis Smith) were the most important, with the first three showing highest incidence in the warm rainy season (October-March). Average percentage infestation of maize at 30 days after emergence in four successive rainy seasons were 11% for maize streak virus, 16% for downy mildew and 26% for stemborers. The multimammate rat was most abundant at the beginning of the cool, dry season (April-May). Maize streak virus attacks were aggravated when there was rainfall in the preceding cool, dry season. High rainfall in the early part of the warm season increased the likelihood of outbreaks of the multimammate rat in the subsequent cool season. Early sowing decreased the intensity of downy mildew and stemborers, but not of maize streak virus. In on-farm trials involving varieties with combined tolerance to maize streak virus and downy mildew, ESR-DMR-W from IITA (Nigeria) produced 32% more grain while CW-1, a locally bred improved variety, produced 31% more than the local unimproved variety.