Ultrathin sections of 2- and of 5-day-old cultured tobacco anthers inoculated at the stage of the first pollen mitosis were examined in the electron microscope in an attempt to identify early structural changes associated with pollen embryogenesis. On both sampling occasions grains were observed with structural features of typical bicellular gametophytes but lacking starch. Some of these grains were atypical in that they possessed numerous small vacuoles in both cells. There was no obvious change in organelle structure in either cell from 2 to 5 d, though there was evidence of an increase in ribosomes and other organelles in the vegetative cell. Gametophytes possessing starch were also present but in relatively low numbers. At 5 d, pollen tubes and grains having structure characteristic of germinating pollen were also encountered. It is concluded that embryogenesis begins after a short period of normal gametophytic differentiation which, during the first 2 d of culture, proceeds at much the same rate as in vivo. Subsequently the rate declines and gametophytic processes gradually come to a halt. Starch formation is suppressed.