The southern root-knot nematode (RKN) [Meloidogyne incognita (Kofoid & White)] is a serious pest of cotton (Gossypium hirsutum L.) with detrimental effects being most pronounced on sandy soils that are also infested with the Fusarium wilt pathogen. Varietal resistance is an effective method of managing the RKN/Fusarium wilt complex. In 1970, a high level of RKN resistance was developed in the germplasm line Auburn 623 RNR, but no commercial cultivar has been developed with this near-immunity level of resistance. The objective of this study was to evaluate the mode of inheritance of RKN resistance in M-315 RNR (M-315), a germplasm line with the Auburn 623 RNR source of resistance, and in M78-RNR, a day-neutral version of the race stock line T78. These lines were crossed with M8, an RKN-susceptible cotton line, and with each other. The parental, F1, F2, and backcross generations of these crosses were evaluated in the greenhouse for RKN reproduction 40 d after planting in a Wickham sandy loam soil that had been infested with either 5,000 or 10,000 RKN eggs per pot. The minimum number of genes conditioning resistance in M-315 and M78-RNR was estimated at two and one, respectively. Mendelian analyses indicated that a two gene, one dominant (Mi1) and one additive (Mi2), model fit the data for M-315. The data from crosses with M78-RNR indicated that it had only the dominant Mi1 gene. These data indicate that the Auburn 623 RNR source of RKN resistance should be easily transferable to commercial cultivars.