Rootstocks have been effective against many soil-borne pathogens in protected tomato production. Rootstocks with heat-stable root-knot nematode resistance may prolong the production season since the root-knot nematode resistance gene Mi-1.2 irreversibly breaks down at soil temperatures above 28 degrees C. The objective of this study was to investigate the effect of soil temperature on root-knot nematode resistance conferred by two genes of tomato, using some commercial tomato cultivars, rootstocks, and PI lines. The response of these genes against Meloidogyne incognita race 2 was studied in two commonly used rootstock cv. Beaufort and Vigomax, in tomato cultivars Astona RN F1 and Simita F1, and in Solanum lycopersicum L. accessions PI126443 and PI270435, known to possess heat-stable nematode resistance, at 24 degrees C and 32 degrees C under controlled conditions. Each plant was inoculated with 1000 M. incognita race 2 second-stage juveniles (J2s) and its response was evaluated 8 weeks post inoculation. The presence of the Mi-1.2 gene was determined with molecular markers. Astona RN F1, Vigomax, Beaufort, PI126443 and PI 270435 which carried the Mi-1.2 gene were resistant to Meloidogyne incognita race 2 at 24 degrees C. The egg masses and J2s were significantly fewer in these lines than in the susceptible Simita F1 at 24 degrees C, and there were no significant differences among resistant plants. In contrast, there were significant differences in the galling index among heat-stable sources and plants containing the Mi-1.2 gene. Simita F1, Astona RN F1 and the rootstocks had a susceptible reaction to M. incognita race 2 at 32 degrees C, but PI 126443 and PI 270435 were resistant. However, at this temperature there were significant differences in the number of juveniles in the soil, the egg mass and the galling index between the heat-stable and the heat-unstable plants.