Conventional tillage (CT) and zero tillage (ZT) farming systems were compared in terms of losses of sediment, nitrogen (N) and phosphorus (P) in surface runoff from watersheds with heavy clay soils in central Texas, U.S.A. CT included several sequential tillage operations that resulted in burial of most of the plant residue from the previous crop. ZT involved no primary or secondary tillage, and crops were planted through the residue from the previous crop. ZT had no effect on runoff amounts but reduced the loss of sediment, N, and P relative to CT. Similar results were obtained for each of the three crops used in the study. The EPIC (Erosion-Productivity Impact Calculator) model successfully simulated the effect of tillage on runoff, sediment, and nutrient loss with the exception of soluble N and P.