Different corn hybrids and soybean cultivars were tested for their allelopathic effects on the subsequent corn yield. Three planting dates were imposed on a mid-season hybrid or cultivar so that the developmental stage effect of corn or soybean could be evaluated. The residues of corn and soybean were either removed or incorporated into the soil. On the average, corn and soybean did not significantly affect the subsequent corn yield. The difference between yields of corn in a continuous corn and corn following soybean was mainly due to the inhibitory allelopathic effect of the previous corn. Maturity type and hybrid of corn, associated with plant size, were the prime factors affecting the subsequent corn yield. Late corn hybrid and mid-season corn hybrid planted on May 15 strongly depressed grain yield of the subsequent corn. For the mid-season corn hybrid, the more progress the developmental stage of corn toward maturity, the stronger the inhibitory effect. Soybean cultivars did not affect the subsequent corn yield. Nevertheless, there was a slight increase in grain yield of the subsequent corn caused by soybean;When the data were compared to 100% for oat control plots, the results indicate that the late corn hybrid obviously reduced the subsequent corn yield and all soybean cultivars increased grain yield of the subsequent corn. Either the corn residue removed or incorporated affected the subsequent corn yield similar to oat. For soybean, either the residue was removed or incorporated, grain yield of the subsequent corn was increased, as compared to oat control plots;Using the Lemna bioassay to detect the allelochemicals in the extracts of soil from the previous corn and soybean plots was unsuccessful. The results from the two years studied were sometimes contradictory. In general the highest extract concentration from both corn and soybean was inhibitory to Lemna growth;A study was conducted to determine if intercropping soybean into corn increased the grain yield of the subsequent corn by means of stimulatory allelochemicals. Three soybean cultivars with three corn to soybean ratios were used. Intercropping did not affect the grain yield of the subsequent corn. Evidence from the study suggests that certain management practices may be recognized and employed to increase yield of the second year corn.