There are four known races of the Southern root-knot nematode, Meloidogyne incognita. Races 1 and 2 do not reproduce on cotton and therefore, are considered non pathogens of cotton; Races 3 and 4 do reproduce on cotton and are considered pathogen of cotton. In tests where soil was infested with a non-virulent race of the nematode 3- or 5- weeks prior to infestation with a virulent race, a root-knot nematode susceptible cotton supported less nematode reproduction than did susceptible plants not challenged with the avirulent race of the nematode. This indicates that some degree of resistance is induced by prior inoculation with a non-virulent race of the nematode. Furthermore, within the limits tested, the degree of induced resistance was directly proportional to the lapsed time between challenge by the non-virulent and virulent races of the nematode.