A quantitative light and electron microscope study of developing and degenerating arbuscules of the mycorrhizal fungus Glomus fasciculatum (Thax. sensu Gerd.) Gerd. & Trappe in Triticum aestivum L. (wheat) and Avena sativa L. (oats) was carried out in order to estimate three parameters during the colonization cycle and to compare these parameters to those in Zea mays L. The parameters are: (1) Vv (f,c), the fraction of the host cell volume (c) occupied by a volume of fungus (f); (2) Vv,(cy, c), the fraction of the host cell volume occupied by host cytoplasm (cy); (3) Sv(p,c) the surface area-to-volume ratio of the host protoplast (p) to the whole host cell. Uninfected cortical cells of wheat had an Sv,(p,c) of 0.11 μm2μm −3 and those of oats 0.10 μm2μm −3. As the fungus penetrates the cell wall, the protoplast invaginates, causing a decrease in protoplast volume and an increase in St. In wheat this increase reached 1.17 μm2μm−3 and in oats, 1.04 μm2μm−3. When the arbuscule is mature, the fungus occupies 35% of the cell in wheat with 20% as branches and 15% as trunk. In oats, the fungus occupies 36% of the cell, with 23 % as branches and 13 % as trunk. In wheat, host cell cytoplasm was initially 39 % and increased to 21.6% and in oats, from 3.6 to 28.8%. These values are similar to those observed in Zea mays. The rates at which the parameters Vv(f,c) and Sv(p,c) changed during arbuscule development and degeneration were similar. The amount of encasement and host cell cytoplasm showed the greatest variation among host species.