Sperm whale social distribution was investigated in the Mediterranean Sea, using data collected during summer surveys from 1997 to 2002. Variations in the size of sperm whale schools/underwater aggregations were assessed using both visual and acoustic data. Individual body lengths were estimated acoustically, using the click inter-pulse intervals. Regional comparisons were undertaken, taking the 41° parallel as a north/south boundary. In the southern region, schools of up to seven sperm whales were sighted and calves were relatively frequent. The animals ranged between 8·6 m and 13·1 m long. In the northern region, school sizes were significantly smaller, with a maximum of three whales sighted at the surface. However, the acoustic survey showed that sperm whales form loose aggregations of up to five animals in certain areas. Whales detected in the north were 12·6 m long on average, and the body size range was relatively small. This summer survey demonstrated a segregation of males, in the north, from larger schools including calves, which seemed to be confined to the southern region.