Membranes from the retina outer segments of squid (Loligo sp.) were isolated by sucrose continuous gradient. In these membranes, phosphatidylcholine and phosphatidylethanolamine were shown to be the most abundant lipids, while rhodopsin was the predominant protein. Steady-state fluorescence depolarization of the probe 1,6 diphenyl-1,3,5-hexatriene was employed to characterize the microviscosity of the photosensitive membranes and of the vesicles prepared from their lipid extract. The lipid fluidity parameter (r0/r-1)-1 in these membranes ranged between approximately 3.8 and approximately 0.95 (0-38 degrees C), while the value of the lipid extract vesicles ranged between approximately 3 and approximately 0.6. No phase transition was observed in the membrane and vesicle preparation in the range of temperatures under study [0-38 degrees C). Photosensitive membranes from cuttlefish (sepia officinalis) gave similar results. The (r0/r-1)-1 value in this species varied between approximately 4 and approximately 1.2 (0-38 degrees C) with a linear dependence on temperature. In these outer segments the bleaching of rhodopsin did not cause any change of the membrane lipid fluidity value. The present results are compared with those obtained in bovine rod outer segments.