Abstract: A natural population of Meloidogyne incognita from field was inoculated on resistant pepper cultivar HDA149，which carried a single dominant nematode-resistant gene Me3. For 15 successive generations，a pair of near-isogenic lines of virulent and avirulent nematode to Me3 was obtained. The fitness cost was analyzed with the reproductive fitness（RF），that Me3 virulent nematodes on the resistant pepper cultivar HDA149 and susceptible pepper cultivar Qiemen. The result indicated that the fitness cost was associated with virulence on susceptible host Qiemen and the fitness value was only 76%. The specialization was detected by the reproductive index of Me3 virulent nematodes on pepper and tomato which contained different resistant genes（Mi，Me）. The Me3 virulent nematodes didn’t produce any root-knots or egg-masses on resistant tomato which with Mi gene，but the virulent nematodes produced egg-masses on resistant pepper which carried other Me genes，it showed that the Me3 virulence had specialization.

Key words: Meloidogyne incognita, Capsium annuum, Me3 gene, virulent population, fitness cost, specialization