<jats:title>SUMMARY</jats:title><jats:p>The present study was designed to test the hypothesis of an oxygen limitation defining thermal tolerance in the European cuttlefish (Sepia officinalis). Mantle muscle organ metabolic status and pHiwere monitored using in vivo31P NMR spectroscopy, while mantle muscle performance was determined by recording mantle cavity pressure oscillations during ventilation and spontaneous exercise.</jats:p><jats:p>Under control conditions (15°C), changes in muscle phospho-l-arginine (PLA) and inorganic phosphate (Pi)levels could be linearly related to frequently occurring, high-pressure mantle contractions with pressure amplitudes (MMPA) of &amp;gt;0.2 kPa. Accordingly,mainly MMPA of &amp;gt;2 kPa affected muscle PLA reserves, indicating that contractions with MMPA of &amp;lt;2 kPa only involve the thin layers of aerobic circular mantle musculature. On average, no more than 20% of muscle PLA was depleted during spontaneous exercise under control conditions.</jats:p><jats:p>Subjecting animals to acute thermal change at an average rate of 1 deg. h–1 led to significant Pi accumulation (equivalent to PLA breakdown) and decrements in the free energy of ATP hydrolysis(dG/dζ) at both ends of the temperature window, starting at mean critical temperatures (Tc) of 7.0 and 26.8°C,respectively. Frequent groups of high-pressure mantle contractions could not(in the warm) or only partially (in the cold) be related to net PLA breakdown in mantle muscle, indicating an oxygen limitation of routine metabolism rather than exercise-related phosphagen use. We hypothesize that it is mainly the constantly working radial mantle muscles that become progressively devoid of oxygen. Estimates of very low dG/dζ values (–44 kJ mol–1) in this compartment, along with correlated stagnating ventilation pressures in the warm, support this hypothesis. In conclusion, we found evidence for an oxygen limitation of thermal tolerance in the cuttlefish Sepia officinalis, as indicated by a progressive transition of routine mantle metabolism to an anaerobic mode of energy production.</jats:p>