The primary current in the Gulf of Mexico is in the form of a loop entering through the Yucatan Channel and eventually leaving through the Florida Straits. It usually transports more than 25 million m3/sec of water at 50 to 200 cm/sec. Although it retains its basic characteristics along the line of flow, it is known to be highly variable in position. Little information on the exact nature of the variations is published. A series of eight cruises of about 2-weeks duration each was conducted by the author over a 30-month period in the different seasons. The primary current was crossed forty times. Five of these cruises supplemented by three others having somewhat differing objectives provided a series of eight cruises in one 16-month period beginning in July 1965. A reasonable sequence of current patterns for the primary current loop is indicated by the observations. The variations in pattern are compared with those indicated by data available from other time periods. The flow is well represented year-around by the topographies of the 22°C isothermal surfaces. This permits a simplified analysis and allows conclusions about the current systems to be drawn from cruises on which only limited data were collected. From July 1965 through December an eastward flow along the coast of Cuba strengthened and became the start of a “spring intrusion” into the gulf. By August 1966 the intrusion had reached 760 km across the gulf, and a “fall spreading” into the west gulf had begun. In July and August 1965 the northern end of the loop became a separate eddy, a detached exterior flow.