Inshore waters of continental North America in the western North Atlantic are populated by only two major geographical floral units. One of these originated and is centered in the tropics and its major inshore northern boundary is Cape Kennedy (formerly Cape Canaveral), Florida. The other originated and is centered in the temperate-cold waters of the North Atlantic and its major inshore southern boundary is Cape Cod. The long coastline between these two boundaries is a vast transition zone occupied either continually or seasonally by the more eurythermal elements of each major flora, and probably there are no species restricted to the transition zone. This extensive zone of overlap of the two floras is readily divisible into several subzones bounded by temperature breaks related to coastal surface currents. The algae of the western North Atlantic can be divided into seven distributional groups, two of which occupy the centers of distribution of the two floras.