Experiments using cultivars with differing degrees of striga resistance were conducted at two sites at Kamboinse in 1988 and at two locations (Kamboinse and Kouare) in 1989 in the Sudan-Savannah region of Burkina Faso. At each site, striga-free (SFP) and striga-infested plots (SIP) were selected. Two factors, location and genotype, were found to be associated additively with yield losses in soils infested by striga. The location effect was probably due to lower soil fertility in the SIP than the SFP plots under farming conditions. Yield losses in SIP relative to SFP ranged from 3·1%, at the experimental station, to 44·2% under farmers' field conditions. The genotype effect was evident at all locations. Depending on the susceptibility of the cultivars, it varied from 3·1 to 36·5% of the mean yield of SFP with an average of 31·4% in susceptible cultivars. The location effect was evident only at Kouare, where SIP plots were under continuous cultivation without appropriate soil fertility maintenance and/or restoration measures. This amounted to c. 19·4% of the mean yield in the SFP. To reduce yield losses in soils infested by striga, it appears to be necessary to grow high yielding, striga-resistant cultivars using agronomic practices which are known to improve soil fertility.