Paleotemperature analyses were carried out on more than 60 Belemnoidea obtained from Mesozoic strata in West and South Australia and New Guinea. One set of data records mean rostral temperatures, and another derived by analyzing successive increments of powdered carbonate from the rostra shows the variations of temperature during ontogenies. The latter probably represent and n i this event would confirm seasonal changes during the Mesozoic. The former demonstrate a cooling from the Jurassic into the Cretaceous and are consistent with the extension of the Albian and Coniacian-Santonian climatic maxima, previously demonstrated in Europe, into the Australian area. The Cretaceous readings dispose of the idea of an ice age in South Australia at that time. It appears that the Belemnoidea were eurythermal through most of their evolutionary history. The paleotemperature results are in accord with a possibly large migration of Australia during the Mesozoic.