The North American populations of the house sparrow, Passer domesticus, introduced from western Europe in the period 1852-1860, show size differentiation in all of 16 skeletal characters of some 1752 specimens over 33 localities. The inter-locality variation as summarized in a principal component analysis shows that the first three components summarize about 70% of the variation in the 16 bony dimensions. Component I represents a gross size factor, bearing strong negative regression relationships with measures of winter temperature, consistent with the empiric model of adaptation implied by the ecogeographic rule of Bergmann. Component II tends to represent a size factor bearing on thermoregulatory dictates of size in body core relative to limbs, consistent with the ecogeographic rule of Allen. These generalities strongly support the idea that size variation in these birds is adaptively organized. Various measures of intrasample and intersample variability show that European and North American populations are identical in intrasample variance for 10 of 12 male characters and 11 of 12 female characters; mean coefficients of variation are essentially identical for all characters. Geographic variation of North American males is less than that of European males, but slightly greater than that of males from England and Germany. Geographic variation of North American females is less than that of European and English-German females. It would appear that constraints on geographic variation of the North American populations are those stemming from the limited number of generations available for selective reassortment of available variation in the period 1852-1962. Multivariate clustering techniques show that a non-hierarchic infraspecific taxonomic structure based on size is developing in the North American populations. The clusters represent current stages in a process of continuing size differentiation and thus are temporal artifacts; nevertheless, they also represent products of the evolutionary process and are of considerable theoretic value.