The Carpinus caroliniana Walter complex (Betulaceae) is a morphologically variable group extending geographically from southeastern Canada to northern Central America. In this study, a series of multivariate procedures has been applied to morphological data obtained from throughout the range of the complex to identify and characterize patterns of regional variation. The methods used include principal components analysis, an a posteriori test procedure (SNK test), and stepwise discriminant analysis. Coefficients of phenetic variation were obtained to estimate the withinand among-population variabilities of subgroups, and multiple regression analyses were used to determine relationships between variation patterns of morphological and environmental variables. These analyses demonstrate patterns of significant morphological differences corresponding to large geographical regions having distinct environmental conditions. Four major morpho-geographic groups are recognizable, these integrating in relatively narrow zones at their points of contact. Within the various groups, which possess character states linked to different ecological roles or survival strategies, the expression of individual characters is often in divergent directions, the changes related to different environmental features. Carpinus caroliniana Walter sensu lato (Betulaceae) ranges throughout the forested region of eastern North America from southern Quebec to western Minnesota, and south to coastal Texas, Louisiana, and central peninsular Florida. It occurs as well in the mountains of both eastern and western Mexico, extending as far south as the northern border of Nicaragua. North of Mexico, the species is usually represented by small, crooked trees occurring in the understory of moist woods, especially on floodplains and slopes adjacent to streams, where it sometimes reaches heights of about 10 meters and trunk diameters of 50 centimeters, although usually it is much smaller. It is found in the United States in a rather wide variety of topographic situations and occurs in vegetation ranging from mixed conifer-hardwoods to oakhickory and southern mixed forests. In Latin America, it grows at relatively high elevations, often in cloud forest vegetation, where it is sometimes a major component of the canopy, attaining trunk diameters of up to one meter and heights of as much as 20 meters. Geographically-correlated variation in the complex was first noted by Fernald (1935), who formally recognized the populations of the northeastern United States, adjacent Canada, and the Appalachian Mountains as C. caroliniana var. virginiana (Marshall) Fern., and those of the southeastern Atlantic and Gulf of Mexico Coastal Plains of the United States as Carpinus caroliniana var. caroliniana. The Latin American segment was given varietal status by John Donnell Smith in 1890 (C. americana var. tropicalis J. D. Smith), and species status by Lundell in 1939. Most members of each of these taxa are easily distinguished on the basis of shape, size, and margin characters of the leaves. However, none of the formal infraspecific or segregate taxa has received wide acceptance, and the entire complex, in most cases, has been treated as a single variable assemblage. Regional variation in a number of characters of the populations of the eastern United States, along a north-south transect extending from Michigan to Alabama, has been studied by Wardell (1976), Wardell and Winstead (1978), and Winstead et al. (1977). These studies demonstrated statistically significant clinal variation in fruit weight, infructescence bract width, wood specific gravity, bract surface area, and the amount of cold needed to initiate bud bursting in the spring. Bract length was found to be shortest at central latitudes. No prior attempt has been made to measure patterns of variation in the complex as a whole to test the validity of the taxonomic treatments discussed above. The purpose of the present study is to describe precisely the patterns of morphological variation of this widespread group, to clarify