Physical characteristics of the Alboran Sea in the western Mediterranean were examined in October 1977 during a survey by ship and aircraft. The main features observed were a distinct oceanic front coinciding with the jet of incoming North Atlantic water and an associated anticyclonic gyre in the western half of the basin. The front was confined to the upper 200 m and was a continuous feature extending from the Strait of Gibraltar 500 km eastward to the prime meridian. Of particular interest was the observed variability of the anticyclonic gyre. In 10 days between two surveys the center of the gyre shifted 50 km westward. This shift may correspond to changes in strength of the North Atlantic inflow as inferred by local wind and the average atmospheric pressure over the western Mediterranean.