ABSTRACTThe effects of different glutaraldehyde‐osmium fixation schedules on the ultrastructure of the vegetative cells from the meristematic regions ofScytosiphon lomentaria(Lyngbye) Link fronds are described. The best overall preservation of cell structure was obtained with a 2 h fixation in 2.5–3.5% glutaraldehyde in 0.1 M cacodylate buffered seawater (pH 7.0), followed after washing by 1 h post fixation in 1% osmium tetroxide. The addition of 1% caffeine to the glutaraldehyde fixative resulted in better retention and spatial localization of the electron dense phenolic deposits within the cells. Particular attention was paid to the effects of the various fixation schedules on the electron‐dense material within the cells and the images obtained were compared with previous accounts of brown algal cells. It is proposed that the term physode should be restricted to the discrete electron dense spherical bodies within the vacuoles and not applied to electron dense material in general. Although the organization ofScytosiphoncells was similar to that previously reported in the Scytosiphonaceae, the organization of the plasmodesmata into pit fields is at variance with previous acc