This study uses an approach derived from models of the private and public characteristics of goods to illustrate (1) the overriding importance of variety attributes in farmers' decisions to allocate area among varieties of maize landraces and (2) the significance of farmers' perceptions of changes in the maize germplasm base in the surrounding community in their choices. Diversity indices and the concept of 'scale', as understood in ecology, are adapted and employed to test hypotheses empirically. Though a case study of maize farmers in Southeastern Guanajuato, the research raises methodological issues for models of variety choice and has policy implications for the potential to conserve maize genetic diversity on farms in Mexico.