In layer IV of the primary visual cortex, in both the macaque monkey and the cat, geniculocortical terminals representing the two eyes are segregated into alternating zones known as ocular dominance bands. Viewed tangentially, in the monkey these bands take the form of a series of branching parallel stripes that run roughly perpendicular to the border of striate cortex. In the cat, the overall ocular dominance pattern consists of irregularly branching, beaded bands that exhibit no predominant orientation. If the striking differences in the appearance of these two patterns reflect important differences in the basic rules governing cortical ocular dominance, then this poses a problem for attempts to formulate general principles of visual cortical organization. However, it has been suggested that the differences in the appearance of the ocular dominance patterns in these two species could result simply from known differences in the boundary conditions of their geniculocortical pathways. This article describes the formulation and testing of a single computational model that accurately predicts the quite dissimilar ocular dominance patterns in cats and monkeys. This model also generalizes to predict the different ocular dominance patterns observed in young and old three-eyed frogs, supporting the notion that the overall pattern of ocular dominance is governed by a common set of rules. The significance of these results is discussed in terms of previous models, which have focused largely on local processes underlying the development of ocular dominance segregation. Although the present model is not a developmental one, it does shed some light on potential mechanisms for establishing retinotopy in striate cortex and on possible developmental relationships between the geniculostriate pathway and intrinsic modularity of the striate cortex.