Dry weight, stem and branch length and node number, and flower and fruit number where appropriate, were recorded through the season on cotton, groundnuts, kenaf and sesame in the central rainlands of the Sudan in 1965. For each sampling date variation within plots and betweenreplicate plots within blocks and treatment x block interaction have been studied, together with the effect of these components on the magnitude of the residual root mean square in the analysis of treatments x blocks. The main components were usually within plot and between replicate plots within block errors. Uncontrolled variation was unacceptably great, and was thought to be in large part the result of irregular surface drainage conditions. It seems more likely to be useful to include surface drainage contrasts in field experiments subject to high intensity rainfall on this impermeable clay soil than to attempt to increase precision alone.