Experimental results and farmer surveys in a Mexican community indicate that farmers' seed selection practices protect the phenological integrity of their maize varieties as they define them, despite numerous factors contributing to genetic instability. Analysis of morphological and genetic data suggests that when subjected to significant gene flow through cross-pollination, ear characteristics are maintained through farmers' selection even though other characteristics may continue to evolve genetically. Because the effects of farmers' selection practices are confined largely to ear characteristics and plant characteristics that are linked to them, their practices appear to offer only limited scope for improving varieties. Farmers' expectations of what they can achieve through seed selection are similarly modest. These findings indicate potentially complementary roles for professional breeders and Mexican farmers in developing methods to improve maize landraces on farms - if farmers themselves perceive benefits from the collaboration.