Callose was detected by fluorescence microscopy in megasporogenesis in all investigated species with mono- and bisporic embryo-sac development. Callose occurs first in the meiotic prophase in the chalazal part of the megasporocyte wall and by the first meiotic metaphase the whole cell is enveloped in a callose-containing wall. Later, there is a marked decrease of callose fluorescence, usually at the chalazal end of the megasporocyte. In Oenothera, where the micropylar megaspore is active, decrease of fluorescence takes place at the micropylar pole of the megasporocyte. Callose appears centrifugally in the cell plates forming eventually the walls dividing the megaspores. It disappears from the walls of the megaspores during degeneration and differentiation.