Abstract Regional and temporal differences in plasma membrane lipid mobility have been analyzed during the first three cleavage cycles of the embryo of the polar-lobe-forming mollusc Nassarius reticulatus by the fluorescence photobleaching recovery (FPR) method, using 1,1′-ditetradecyl 3,3,3′,3′-tetramethylindocarbocyanine iodide (C 14diI) as a fluorescent lipid probe. During this period of development the lateral diffusion coefficient of membrane lipids is consistently greater in the vegetal polar lobe area as compared to the animal plasma membrane area (on average 30%), demonstrating the existence of an animal-vegetal polarity in plasma membrane properties. At third cleavage, the differences between animal and vegetal plasma membrane region become even more pronounced; in the four animal micromeres the diffusion coefficient ( D) and mobile fraction (MF) are 2.9 ± 0.2 × 10 −9 cm 2/sec and 51 ± 2%, respectively, while in the four vegetal macromeres D = 5.0 ± 0.3 × 10 −9cm 2/sec and MF = 78 ± 2%. Superimposed upon the observed animal-vegetal polarity, the lateral diffusion in the polar lobe membrane area shows a cell-cycle-dependent modulation. The highest mean values for D are reached during the S phase (ranging from 7.0 to 7.8 × 10 −9 cm 2/sec in the three cycles measured), while at the end of G 2 phase and during early mitosis mean values for D have decreased significantly (ranging from 5.0 to 5.9 × 10 −9 cm 2/sec). Diffusion rates in the animal membranes of the embryo are constant during the three successive cell cycles ( D = 4.3−5.0 × 10 −9cm 2/sec), except for a peak at the S phase of the first cell cycle ( D = 6.0 × 10 −9cm 2/sec). These results are discussed in relation with previously observed ultrastructural heterogeneities in the Nassarius egg plasma membrane. It is speculated that the observed animalvegetal polarity in the organization of the egg membrane might play an important role in the process of cell diversification during early development.