The late prehistoric and protohistoric population of eastern Sonora, Mexico, has been described on the basis of reports by early Spanish explorers, as being organized into modified chiefdoms called ``statelets.'' The development of these small, regionally discrete political units has been seen as the result of Casas Grandes influence. Archaeological data on late prehistoric settlement patterns gathered during an intensive survey of the Valley of Sonora and presented here verify that statelets did exist. Data on earlier settlements and settlement-pattern changes, however, are interpreted as meaning that statelets developed without external influences. A growing population and increasing local exchange are proffered as the underlying causes for statelet development. Contact and trade with neighboring groups did exist, but probably were consequences rather than causes of changes evident in the settlement patterns.