Greenhouse pot studies conducted previously by several laboratories identified certain primitive accessions of exotic cottons that show potential as sources of reniform nematode resistance for breeding programs aimed at developing reniform nematode resistant cultivars. In 2001, the first field experiment was conducted to directly examine the ability of the most promising of these accessions to reduce reniform nematode populations in the field. Accessions were planted in a field with a sandy clay loam soil 7 km north of Weslaco, Texas, in a randomized complete block design on 102-cm-wide beds during the first week of March, 2001, and soil cores for analysis of nematodes, roots, soil moisture, and texture were taken to a depth of 122 cm in March as well as during the first week of July, just prior to harvest of agronomic genotypes in the same field. The mean reniform nematode population density at planting, when averaged from the surface to 120 cm, was 3 Baermann funnel-extractable nematodes/g soil (estimated equivalent to 3,690 nematodes/pint by sugar flotation). In July, nematode populations at 15-cm increments from the soil surface to the 120-cm depth revealed that reniform nematode populations under G. arboreum A2-87, G. herbaceum A1-17, and the G. barbadense accessions GB-13, GB-49 and TX-110 had increased about 4-fold, did not differ significantly from the susceptible control, and thus did not exhibit appreciable resistance to the nematode. Statistically significant (P = 0.05) nematode population suppression was achieved only for GB- 264, and the level of suppression measured (48%) was not considered agronomically useful. Follow-up experiments conducted under microplot and growth chamber conditions confirmed that these accessions were able to exhibit moderate to high levels of resistance in a sandy loam soil and in a sand-based potting mix but not in soil taken directly from the Weslaco field site. Loss of resistance expression was unrelated to nematode genotype, moisture, nematode extraction technique, ambient temperature or light quality. Possible remaining explanations included downward movement of nematodes in the field, potent antagonists in the upper soil profile in the field, and abnormal root-growth in pot and microplot experiments.