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Mosaic disease of cassava is one of the most important factors limiting production in East Africa. The disease can be controlled by the use of healthy planting material. Plots of carefully selected, apparently healthy cassava of several cultivars of differing genetic constitution were established and successfully kept free of disease by inspection and roguing. The rate of spread of mosaic disease into mosaic-free plots over a five-year period was consistently very low (less than 2% and most frequently less than 1%) irrespective of cultivar type or size of plot (0.02-1.00 ha), which clearly indicates that mosaic disease can be effectively controlled in Kenya by the use of mosaic-free propagation material. Comparative yield data indicated that yields of local cassavas are at least equal to those of improved resistant cuitivars and suggest that agronomic evaluation of Kenya cuitivars would be of great value.