In recent years, the use of fast-freeze fixation followed by freeze-substitution has been shown to be the procedure that best satisfies the ultrastructural preservation of cellular components due to the rapidity of the fixation procedure and the reduction of artifacts compared to chemical fixation. When these techniques were used to study the fine structure of axenically cultured trophozoites of Acantamoeba castellanii, an improved preservation of the whole cell was observed. The ground substance of the cytoplasm is densely packed with fibrogranular material which is frequently removed with the use of conventional techniques. Also, the ultrarapid physical stabilization allows the visualization of fusion and fission processes of cytoplasmic vacuoles and vesicles. The nuclear structure and cytoplasmic microfilaments as well as membranous structures were clearly identified. Low temperature techniques combine the advantage of fast-freeze fixation for the physical stabilization of organic molecules and their stabilization by the substitution medium at low temperature giving rise not only to a better preservation of the cell ultrastructure but providing a favorable basis for immunocytochemistry at the electron microscopy level.