AbstractIn order to determine the best conditions to carry out quantitative ultrastructural studies in plant specimens, five different fixation techniques, including some of the most reported electron microscopy fixatives (glutaraldehyde‐paraformaldehyde, osmium tetroxide, potassium permanganate), were assayed in onion root meristems to check their ability to induce morphometric changes in Golgi apparatus ultrastructure. Although the parameters evaluated showed in all cases the same tendency, values obtained after permanganate fixation were always higher than those found after aldehyde techniques (especially aldehyde‐osmium). Aldehyde followed by osmium fixation appears as the most indicated fixation method when accurate quantitative ultrastructural studies are to be develo