The root-knot nematode Meloidogyne exigua is a huge threat in major Arabica coffee growing areas in Latin America. The development of nematode-resistant coffee trees constitutes the most promising option for controlling the pest. Coffea arabica resistance to M. exigua is controlled by a simply inherited major gene, called the Mex-1 gene. The objective of this study was to determine the level of expression (complete or incomplete) of the Mex-1 gene within homozygous or heterozygous C. arabica genotypes, and the stability of that expression under field conditions. Resistant and susceptible pure line cultivars were compared with clones of hybrid cultivars derived from crosses between resistant and susceptible lines. The results under controlled conditions in a greenhouse revealed that reproduction of the nematode was significantly higher on hybrid cultivars than on resistant pure line cultivars, but much lower than on a susceptible pure line cultivar. The same result was confirmed under field conditions. There were fewer galls in the hybrid cultivar than in the susceptible pure line cultivar. A significant number of the galls in the susceptible cultivar were large, whilst in the hybrid cultivar, only small galls were found. A histological analysis of galls did not reveal any differences in patterns of giant syncytial cells between susceptible and homozygous or heterozygous resistant genotypes. After being monitored for 4 years, a M. exigua field population was multiplied by a factor 14 on a susceptible cultivar and by a factor 1.9 on the hybrid cultivar. We concluded that Mex-1 could have incomplete dominant expression that allowed nematode penetration, but inhibited the durable reproduction of the nematode. Finally we introduce the "gall diameter" as a new parameter that could be used to characterize the intermediate resistant phenotype for breeding purposes in coffee. ©2006 Elsevier Ltd. All rights reserved.