The well known benefits of grass fal1ow on subsequent crops in the savanna zone of the tropics are apparently due to an improved nutritional status rather than to any significant improvement in the physical properties of the surface soil. Infiltration rates were measured in grass fallow rotations as a test for changes in physical properties. Infiltration rates increased during the fallow period in relation to the square root of lime. However, most of the increase was eliminated during the first seedbed preparation and all the increase was eliminated by the end of the first cropping season. The increase in infiltration rates in fallow was primarily due to earthworm activity which produced only fragile, water conducting pores and channels but apparently no durable aggregation in the sandy surface soil.