The crystal-liquid crystal-isotropic melt phase transitions of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) from muscle tissue of five species (actinia Metridium senile fimbriatum, mussel Crenomytilus grayanus, sea-urchin Strongylocentrotus intermedius, starfish Distolasterias nipon and the ascidian Halocynthia aurantium) of marine invertebrates, collected in winter at 0 degrees C and then acclimated to 18.5 degrees C for 5 days, were studied by differential scanning calorimetry and polarising microscopy. To elevate temperature from 0 to 18.5 degrees C, we used the rate of 4.5 degrees C/h. Although phase transitions of both phospholipids from animals collected in summer occurred already at temperatures below -1.7 oC (minimal temperature of seawater in winter), compensatory mechanisms resulted in a decrease by 29-43 oC in the phase transition temperature of PE in winter. Thermotropic behavior of PCs changed in various trends. However, the total heat of their phase transitions always decreased in winter compared with summer. For all species, except the mussel, the time of warm-acclimation was insufficient to adjust the thermotropic behavior of either phospholipid. Nevertheless, the unsaturation index decreased to achieve summer values, due primarily to decreased proportions of eicosapentaenate and docosahexaenate. The accumulation of arachidonate, during warm-acclimation, might be connected to the signalling properties of n-6 eicosanoids. Absence of effective homeoviscous mechanisms suggests that most of the studied marine invertebrates have very limited capacity to survive an acute temperature elevation, e.g. at the appearance of thermal currents.