The impact of 14 years of continuous conventional (CT) or no-till (NT) cultivation on surface soil structure and crop yields was examined on a weakly structured silty loam soil at Grafton in N.S.W. The annual soybean yields of the NT treatme between 1981 and 1985 were consistently less than or equal to those resulting from CT with an average of 2.46 t ha-1 and 2.82 t ha-1, respectively, for the two treatments. However, CT was unable to sustain the greater yield, and from 1987 onwards the yields of the NT treatments have typically been greater than those of the CT with averages of 2.14 t ha-1 and 1.67 t ha-1, respectively. During the earlier years of the trial, soil porosity and crop yields were not greatly affected by the different tillage techniques. During later years and at the end of the trial, however, soil porosity and structural stability were greater under NT. Increased soil macroporosity (saturated water content of 0.61 for NT vs 0.40 for CT) and structural stability (dispersed silt + clay contents of 10% for NT vs 30% for CT) under long term no-till cultivation were consistent with higher saturated hydraulic conductivity (189 for NT vs 23 mm h-1 for CT), higher infiltration and lower run-off under rainfall, increased plant available water (12.5% for NT vs 10.5% for CT), water use efficiency, and crop yields. The improvement in soil structure observed under NT is associated with the significant increase in surface soil organic carbon contents (3.37% for NT vs 1.67% for CT) and is shown to be the major contributor to the sustained improvement of crop yields.