Heat-flow measurements were made in DSDP boreholes at five sites in the Mediterranean and Aegean seas. AbnormalUy high heat flow was observed in the Algero-Provençal Basin, and in the Tyrrhenian and Aegean seas. Much lower heat flow was observed through the Central Ionian Sea and on the Florence Rise southwest of Cyprus. The heat-flow data from the DSDP holes are in excellent agreement with nearby conventional heat-flow values. High heat-flow through the Tyrrhenian and Aegean seas suggests, but does not require, that these basins are back-arc basins associated with the Calabrian and Cretan island arcs. The explanation of high heat flow through the Algero-Provençal and Balearic basins, and of low heat flow through the entire eastern Mediterranean Sea, may be related to thermal and chemical differences in, and horizontal and/or vertical displacements of, the asthenosphere beneath the converging African and Eurasian plates.