The change of membrane fluidity in rockfish (Sebastes schlegeli) phagocytes during respiratory burst was investigated. Fluorescence polarization (FP) was used as a measure of membrane fluidity, and 1-(4-trimethylaminophenyl)-6-phenyl-1 .3 ,5-hexatriene (TMA.-DPH) was used us a fluorescent probe. The significantly higher FP values in phagocytes stimulated With zymosan or phurbol myristate acetate (PMA) than unstimulated control phagocytes suggests that membrane fluidity of phagocytcs is decreased during the respiratory burst. The faster decrease of FP value in PMA stimulated phagocytes than in zymosan sumulated phagocytes may be due to bypass of the receptor-mediated stages of functional modulation. which is needed in zymosan stimulated phagocytes.