in order to find suitable conditions for the fixation of fish muscles for electron microscopy, effect of the fixative concentration to the musculature of the freshwater and saltwater fish was examined. Carp and horse mackerel were used as the sample of the freshwater and saltwater fish, respectively. Muscle samples were prefixed in 3 % glutaraldehyde-sodium cacodylate buffer (pH 7.2). Following the postfixation in 1 % Os04-sodium cacodylate buffer, muscles were dehydrated in ethanol and

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... in TAAB 812. The molarity of sodium cacodylate buffer was set to 0.05 M, 0.1 M or 0.2 M for the freshwater fish, and to 0.1 M, 0.2 M or 0.3 M for the saltwater fish, in both prefixation and postfixation Muscle samples were block-stained in 1 % uranyl acetate solution after postfixation. Ultrathin sections were examined under electron microscope. The results show that the most suitable concentration of the fixatives is 0.1 M and 0.2 M for the freshwater and for the saltwater fish muscles, respectively. Use of the fixatives lower than the above strengths induced expansion of the mitochondria or the sarcoplasmic reticulum and broadening of the myo fibrils in both fresh and salt water fish muscles. On the other hand, use of the fixatives stronger than the above induced deformation of the mitochondria, and obscuration of the filament structures.