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We have compared the mobility of a fluorescent lipid analogue and of fluorescently labeled membrane proteins at the animal and vegetal poles of the egg of the sea urchin Paracentrotus lividus. Translational diffusion coefficients have been measured by fluorescence microphotolysis (“photo-bleaching”) on the egg which was rotated on its poles. Lipid and protein diffusion coefficients averaged 0.8 μm2/sec and 0.04 μm2/sec, respectively at both animal and vegetal pole of the egg. Substances which were known to animalize (Zn++) or vegetalize (Li+) the sea urchin egg had no significant effect on protein diffusion coefficients.