Resistance to the parthenogenetic root-knot nematode Meloidogyne incognifa is controlled in tomato by the single dominant gene Mi, against which virulent pathotypes are able to develop. Isofemale lines (i.e., families) were established from a natural avirulent isolate of M. incognita in order to study the genetic variability and inheritance of the nematode virulence. From the progeny individual females, the production of egg masses on the root system of the Mi-resistant tomato 'Piersol' was analyzed in artificial selection experiments. A family analysis revealed, after two successive generations, a strongly significant variation between the 63 isofemale lines tested, and the results obtained for the mothers and their daughters were also significantly correlated. These results together clearly demonstrate the existence of a genetic variability and inheritance for this character. In a second experiment, a four-generation selection was performed on 31 other isofemale lines. The results revealed a significant response to selection apparently limited only to the two families able to produce, in first generation, a significant minimal egg-mass number on the resistant cultivar.