Progress in common bean breeding requires the exploitation of genetic variation that is present among races or through introgression across gene pools. Iberian Peninsula is considered as a secondary center of genetic diversity (Santalla et al., 2002). Introgression from the Middle American to the Andean gene pool appears to be relatively common in Andean zones, while Middle American accessions from the Iberian Peninsula exhibit evidence of introgression from Andean beans (Chacón et al., 2005; Paredes and Gepts, 1995). This introgression was assumed to result from spontaneous outcrossing in farmers' fields, based on segregation found previously in farmers' varietal mixtures. Studies employing allozymes and DNA-based markers have revealed dozens of instances of natural introgression in plants, and morphological intermediary and molecular confirmation of introgression go hand by hand. The presence of crop-specific alleles in intermediate populations can help to provide strong evidence for a history of hybridization. The objective of the investigation presented here was to quantify the degree of spontaneous introgression on the phenotype of Mesoamerican landraces.Research was supported by Plan Nacional I+D+I (AGF93-0756-C02) and INIA (RTA03-048-C2-2) projects from the Spanish Government. The authors thank the Xunta de Galicia for funding.Peer reviewed