A collection of 98 breeding Mexican Ducks (Anas platyrhynchos diazi) was made in Mexico from six areas between the United States border with Chihuahua and Lake Chapala, Jalisco, in order to study geographic variation. Plumage indices showed a relatively smooth clinal change from north to south; northern populations were most influenced by the Northern Mallard (A. p. platyrhynchos) phenotype. Most samples were fairly uniform; that from the Río Conchos area in northeastern Chihuahua was noticeably more variable than the others. Soft part coloration is described. Weights varied from 849 to 1,243 g in males and from 647 to 1,264 g in females. Measurements of total, wing, and culmen lengths and bill width were usually significantly larger in males at any one site, but showed no regular geographic trends. Hybridization between platyrhynchos and diazi phenotypes may or may not be increasing in the middle Rio Grande and Río Conchos valleys; available data are insufficient to decide. Introgression of the Northern Mallard genome into Mexican diazi populations may be largely historical owing to recent reductions in the number of migratory Mallards reaching Mexico. A spring 1978 aerial census yielded an estimate of 55,500 diazi-like birds in Mexico. Populations of diazi appear to be as large as the available habitat allows; management should be directed towards increasing and stabilizing the nesting habitat; and the stability of the zone of intergradation should be investigated.