|Reniform nematode, Rotylenchulus reniformis Linford & Oliveira, is one of the most destructive nematode pests of cotton (Gossypium spp.), and upland cotton (G. hirsutum L.), the most common cultivated type, is lacking in resistance. Differing levels of resistance to reniform nematode has been reported in several species of Gossypium. Our objectives in the first part of this research were to compare levels of resistance in two G. hirsutum wild accessions (TX 245 and TX 1419) with those germplasm lines with resistance derived from G. longicalyx, a wild cotton relative that has been reported to be immune, and to determine if resistance in G. hirsutum is a heritable trait. We evaluated F2:3 lines from crosses derived from TX 245 and TX 1419 and an adapted susceptible genotype for resistance to reniform nematode in the greenhouse using LONREN-1, a highly resistant G. hirsutum type with resistance derived from G. longicalyx for comparison. There were no differences among the F2:3 lines for vermiform reproduction. Zero heritability of resistance was determined within these populations, and LONREN-1 resistance was far superior. In the second part of this study, F2:3 lines derived from a cross between LONREN-1 and an adapted susceptible line were tested for reniform nematode resistance. Twenty-one of 100 F2:3 lines were found to be highly resistant and not different from the LONREN-1 parent. We concluded that the previously observed resistance in G. hirsutum was not repeatable, but resistance derived from G. longicalyx was highly heritable and repeatable. The aim of the second part of this research was to evaluate the impact of age and condition of seedlings on reniform nematode reproduction in upland cotton by investigating the relationship between the seedling vigor and genetic resistance to reniform nematode. Seeds of two cultivars (PM 1218 and FM 966, susceptible), two wild accessions (TX 245 and TX 1419, previously showing erratic results with regard to resistance), and LONREN-2 (resistant) were exposed to adverse stress conditions (accelerated aging) to simulate reduced seedling vigor caused by time and improper storage. Genotypes were then tested for reniform nematode resistance by evaluating nematode reproduction, to determine the possible effects on seedling vigor on nematode reaction. Plant height was recorded once a week during the evaluation period. Dry and fresh weights of roots and shoot dry weights were measured at the end of the study. Results showed that germination rate and seedling vigor decreased with increasing length of exposure time to adverse conditions. Reniform nematode numbers for each genotype were not significantly different due to accelerated aging. Consequently, there was no direct relationship between the age and condition of the cotton seedlings and their response to reniform nematode. The objective of the third part of this research was to evaluate the potential use of Simple Sequence Repeats (SSR) to monitor upland cotton populations for resistance to reniform nematode. This was performed by molecular screening of 38 out of 100 F2:3 lines of the cross LONREN-1 × FM 966 in addition to a control group that included both of the parents, LONREN-1 and FM 966 as resistant and susceptible controls, respectively. The SSR markers amplified most of the 38 DNA samples showing that phenotype-based resistance determined in our previous study was reliable except some unexpected results which is remained for further evaluation. Results also show that marker-assisted screening can be easily applied to analyze upland cotton populations for reniform nematode resistance and to select good candidates for further breeding studies.