Corn–soybean [Zea mays L.–Glycine max (L.) Merr.] crop rotation increases yields of both crops, a phenomenon known as the rotation effect. Plant–parasitic nematodes can decrease corn yield, and this study was conducted to determine their role in the rotation effect for corn. Th is study was conducted at a long-term research site that included crop sequence treatments in 1 to 5 yr of corn monoculture following 5 yr of soybean, and continuous corn monoculture since 1982. Granular nematicides were applied to half of each plot to minimize nematode populations as a way to determine their role in the rotation effect. Populations of soybean cyst nematode (SCN, Heterodera glycines), decreased rapidly in corn monoculture (P ≤ 0.05) and Xiphinema (dagger nematode)densities were small—averaging 5 nematodes 100 cm–3. Averaged across crop sequences, nematicide applications increased (P ≤ 0.05) corn yield 3 to 11% and decreased Pratylenchus (lesion nematode) and Helicotylenchus (spiral nematode) populations. Pratylenchus and Helicotylenchus increased (P ≤ 0.05) in corn with Pratylenchus oft en increasing incrementally as years in corn increased, but Helicotylenchus populations generally greater in continuous corn than most other sequences. Yield decreased (P ≤ 0.05) in monoculture, particularly for first through third year corn. Nematicide was more effective in increasing yield in third and fifth year corn than other sequences. Pratylenchus and Helicotylenchus populations corresponded negatively with yield in regression models (P ≤ 0.05) explaining 36 to 42% and 10 to 19% of variation in yield, respectively. Results suggest nematodes, particularly Pratylenchus, contributed to declining corn yield in monoculture.