To study virulence and aggressiveness in root-knot nematodes on cultivars of potato (Solanum tuberosumL., four isolates ofMeloidogyne haplaChitwood race A, one ofM. haplarace B, three ofMeloidogyne chitwoodiGolden et al., and two ofMeloidogyne fallaxKarssen were evaluated on 10 commercial potato cultivars under semisterile conditions in Petri dishes. Virulence and aggressiveness were assessed in terms of nematode reproduction by egg-mass index and reproduction factor, estimated by the number of egg masses and juveniles produced, respectively, divided by the number of juveniles inoculated. Significant interaction for both parameters was revealed between species ofMeloidogyneand potato cultivars. OnlyM. haplashowed significant isolate-by-cultivar interaction, which was predominantly caused by theM. haplarace B isolate Hh. This indicates variation in virulence and suggests the occurrence of different genetic factors for virulence and resistance inM. haplaisolates and potato cultivars, respectively. Despite large differences, the observed levels of resistance were too low to be of practical meaning for breeding, with the exception of resistance to isolate Hh. No significant differences were obtained between isolates ofM. chitwoodiandM. fallaxor isolate-by-cultivar interaction, indicating neither variation in aggressiveness nor in virulence in the isolates used. A comparative greenhouse experiment gave comparable results for egg-mass index but contradicting results for reproduction factor, most likely because of differences in life cycle, which appeared to be shorter inM. fallaxthan inM. haplaandM. chitwoodi. The Petri-dish method proved to be accurate for virulence studies of root-knot nematodes on potato culitvars.Meloidogyne fallaxwas most aggressive on potato, followed byM. chitwoodi,M. haplarace A, and finally byM. haplarace B.Key words: aggressiveness, biadditive model, biplot, plant breeding, resistance, virulence.