Variability of marine faunas in Middle Pennsylvanian rocks in central Colorado, as generally of modern marine faunas, increases with increasing depth and distance from shore in shallow water. Less than 3 genera of brachiopods lived nearshore, whereas 10 were present in water computed to have been 22 meters deep, about 5 kilometers offshore. Foraminifers increase from 1 or 2 irregularly formed types nearshore, to 6–8 types in offshore areas, and foraminiferal density increases sharply 3l/2 kilometers offshore at a depth of about 15 meters. Several genera of brachiopods, 3 types of foraminifers, and phylloid algae show a distribution closely related to depth of water and distance from shore. On the basis of significant changes in genera of brachiopods, types of foraminifers, general faunal diversity, and foraminiferal density, three distinct biotopes can be recognized. These biotopes were occupied by the euphemitid, dictyoclostid-Composita, and fusulinid fossil communities.