There is a large number of algae containing much water and mucoid substance in there
bodies. As they thrive in sea water，a re susceptible to the osmolarity of fixative solution, and it is said that fixation of the various structures in their cells are difficult. This report, seeking the optimum fixation method to observe in details and correctly the form of the tissue，organella and the cell of Undaria pinnatifida，3 kinds of fixation methods，viz. single fixation of osmic acid employed since olden times，mixed fixation of osmic acid and glutaraldehyde，and double fixation of prefixation by glutaraldehyde and postfixation by osmic acid have been tried，compared and studied.
Materials are healthy lamina of Undaria pinnatifida collected on the coast of Onomichi in April，1978. After fixing the material with the respective fixing solution，alcohol dehydration，and after embedding with epoxy resins making them in ultra thin sections，double staining by Millonig solution and uranyl acetate solution，has been applied，and observed
with electron microscopes (HS-7 type and JEM 100-B type).Findings were that single fixation by osmic acid proved effective for observation of nucleus. nucleolus，vacuole, vesicle，ground substance and cell wall，while unfit for observation of chloroplast, mitochondria and Golgi body. With mixed fixation of osmic acid and glutaraldehyde，
vacuole，ground substance and cell wall can be observed very well. however，on other organella，electron density was low, and the structure is obscure. It can be said
that with glutaraldehyde pre-fixation，osmic acid post-fixation, fixation effect and electron staining effect of every organella was good showing that this is the most optimum fixation method.