The pH and d-lactate of the haemolymph from the scorpion, Opistophthalmus latimanus, were measured at different ambient temperatures. The oxygen affinity and molecular cooperativities of haemocyanin in its native and dialysed form were determined when exposed to different ambient temperatures. D-lactate concentration in native haemolymph increased significantly from 0.31 mmol/l at 7 °C to 1.93 mmol/l when haemolymph was exposed to 25 °C for 6 hours but decreases to 0.32 mmol/l at 37 °C The extinction coefficients of O. latimanus haemocyanin at 240 nm and 278 nm were 2.41 and 11.26, respectively, and correspond well with similar data on the crustacean Limulus polyphemus. The subunit cooperativity (n₅₀) of the native haemocyanin molecule increased linearly from n = 3.00 to n = 5.34 at 37 °C. A d-lactate effect on n₅₀ between pH 7.40 and 7.87 could be demonstrated while the heat of oxygenation (ΔH) was -127.64 kJ/mol between 17 °C and 25 °C Like other scorpions investigated, haemocyanin of O. latimanus exposed to wide temperature regimes show a strong temperature dependence of oxygen affinity (as P₅₀) and cooperativity (as n₅₀).