Fermentation of the sugars in bread dough produces carbon dioxide which diffuses through the dough matrix into the gas cell nuclei formed during dough mixing. As a result, the void fraction of the dough increases and dough density decreases. It is shown in this paper that low intensity ultrasound can be used to monitor changes in the void fraction of this opaque material, thus providing real-time information on changes in the structure of the dough during fermentation. Doughs were mixed under two different mixer headspace conditions, and the ultrasonic velocity and attenuation measured as a function of fermentation time. The results of the ultrasonic experiments were compared with changes in dough density (measured independently but under the same experimental conditions). As fermentation time increased (and gas cells expanded), the ultrasonic velocity decreased and the attenuation increased. At early fermentation times, a substantial drop in velocity was observed before the density changed appreciably, indicating that yeast activity has two independent effects on dough properties: modifying the elasticity of the dough matrix and expanding the gas cells. Ultrasound therefore has the potential to provide novel information on technological issues of critical importance to the cereals’ processing industry.