The root-knot nematode (Meloidogyne incognita [Kofoid and White] Chitwood) is a sedentary endoparasite that retards growth and development of cotton (Gossypium hirsutum L.). Some breeding lines of upland cotton developed in our research program exhibit high levels of resistance to root-knot nematode (RKN). Interestingly, infective juvenile RKN penetrate young roots of these resistant lines in the same manner as observed on susceptible plants with formation of small immature galls. However, development of the nematode beyond a swollen J2 stage is inhibited in the resistant plants by 8 to 10 days after inoculation. These observations suggest that some resistance mechanism is initiated in the resistant line following penetration of the nematodes. In this study, we asked whether root gene expression differed between resistant and susceptible plants of semi-isogenic lines (BC2) early after penetration of the nematodes (8-10 days post-inoculation). Sodium dodecylsulfate polyacrylamide gel electrophoresis of total protein extracts revealed the presence of a 14,000 mw polypeptide specifically associated with the RKN-inoculated resistant plants. Because this polypeptide was absent from control (noninoculated) resistant and susceptible plants, its synthesis appears to be induced in the resistant line upon infection with RKN.