After more than half a century of research on the effect of soil pH and aluminum toxicity on corn (Zea mays L.) growth, there still is no general agreement on whether soil pH or some measure of potential aluminum toxicity, such as aluminum saturation, should be the criterion used to estimate the likelihood of lowered yields in acid soils. Corn was grown for two years in two acid (pH 4.7) Ultisols (Wharton soil, and Murrill soil) in Pennsylvania with lime rates up to 16 metric tons per hectare. Grain yield was significantly reduced (p < 0.05) when the pH was less than 5.5 in the Wharton soil, whereas it was not significantly reduced in the Murrill soil until the soil pH was less than 4.9. In both soils the relative grain yield was lowered to 90 percent of maximum when the aluminum saturation exceeded 12 percent. These results indicate that aluminum saturation was a more accurate indicator than soil pH for predicting corn yield reductions in these acid soils. It is suggested that the low aluminum saturation in the Murrill soil at soil pH values of less than 5.0 was due to the very high available phosphorus level in this soil.