The nematodes found most often associated with corn are root lesion, Pratylenchus spp., and spiral, Helicotylenchus spp, nematodes. All but two corn samples submitted to the UW Plant Disease Clinic for nematode diagnosis in 2002 were positive for root lesion nematodes. Three species were most common: P. penetrans, P. scribneri, and P. neglectus. We use a damage threshold of 200 root lesion nematodes per 100 cm3 soil at the time of planting for making recommendations to growers. When samples are submitted in the fall prior to planting corn it is necessary to adjust the counts to account for overwinter mortality. Spiral nematodes, Helicotylenchus spp., are found in almost every corn field in Wisconsin. Spiral nematodes are less pathogenic to corn than root lesion and are considered pests only when population densities exceed 800 per 100 cm3 soil. Both root lesion and spiral nematodes occur in every soil type and have a sufficiently broad host range to be a threat in any rotation scheme. Most of the samples submitted to the UW Plant Disease Clinic represent problem fields and testing for nematodes is often the last resort. The fact that root lesion and spiral nematodes cause very nonspecific symptoms such as slow growth rate or stunted root systems makes diagnosis in the field impossible. For an accurate diagnosis, it is essential to collect a soil sample that represents the affected area.. Since nematodes are likely to be present, and their pest status estimated only by the numbers recovered, several iterations of sampling may be necessary to conclude they are the cause of the problem.