A survey was conducted in southern Tanzania to determine the relationship between above?ground symptoms of cassava brown streak virus (CBSV) infection and root necrosis which is often associated with the disease. CBSV symptoms were found in 62 of the 64 fields sampled. Seventy-nine per cent of plants with above-ground symptoms of CBS also exhibited root necrosis, compared with only 18% of plants with no visible symptoms of CBSV. No fungi were isolated on acidified water agar from more than half the root samples taken from plants with root necrosis. Where fungi were isolated they represented a range of common soil-borne fungi, and no single fungus predominated. No evidence could therefore be found to indicate a cause of root necrosis other than brown streak virus. Some varietal differences in response to CBSV were apparent from the survey and at least one variety developed above-ground symptoms but not root necrosis.