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A method to develop erodibility prediction equations was developed and tested using published data. Published sediment yield data for 21 California forest soils developed using laboratory plots subjected to simulated rainfall were used to develop an equation that explains 90% of the variation in sediment yield from the plots. The predicted sediment yield values can be considered an index of relative interill erodibility because only soil type varied among the plots. Sediment yield was a function primarily of soil texture for soils with less than 50% Si+Cl content. Beyond 50% Si+Cl content, the erodibility of the soil related more to the type of clay: kaolin or smectite. The hypothesis proposed to explain the shape of the curve is based on aggregate strength under rainfall. A second hypothesis that increased silt and clay increases the degree of aggregation of the soil as a whole is rejected. Soils with a relatively high smectite clay content had reduced aggregate strength and higher sediment yield when exposed to rainfall than soils with a relatively high kaolin clay content. The relationship of texture and clay mineral content to erodibility should be general and applicable to other soils.