Root-knot nematode resistance of F~ progeny of an intraspecific hybrid (Lycopersicon peruvianum var. glandulosum Acc. No. 126443 × L. peruvianum Acc. No. 270435), L. esculentum cv. Piersol (possessing resistance gene Mi), and L. esculentum cv. St. Pierre (susceptible) was compared. Resistance to 1) isolates of two Meloidogyne incognita populations artificially selected for parasitism on tomato plants possessing the Mi gene, 2) the wild type parent populations, 3) four naturally occurring resistance (Mi gene)-breaking populations of M. incognita, M. arenaria, and two undesignated Meloidogyne spp., and 4) a population of M. hapla was indexed by numbers of egg masses produced on root systems in a greenhouse experiment. Artificially selected M. incognita isolates reproduced abundantly on Piersol, but not (P = 0.01) on resistant F~ hybrids. Thus, the gene(s) for resistance in the F~ hybrid differs from the Mi gene in Piersol. Four naturally occurring resistancebreaking populations reproduced extensively on Piersol and on the F~ hybrid, demonstrating ability to circumvent both types of resistance. Meloidogyne hapla reproduced on F1 hybrid plants, but at significantly (P = 0.01) lower levels than on Piersol.