Initially homogeneous suspensions of bubbles in basaltic magmas and lava flows may become unstable and form rising waves or layers of bubbles. We derive a set of model equations for the two-phase (bubble-liquid) system and present the results of a linear stability analysis and numerical simulations. Periodic vesicle layers are preserved in Columbia River flows with a spacing of ~1 m; the prediction of the stability analysis combined with a simple model for solidification of the lava suggests that vesicle layers may represent waves of bubbles formed within the flow. The spacing of layers is determined by a balance between the growth of bubbles and hydrodynamic self-diffusion. Vesicular layers preserved in Columbia River flows may thus form by hydrodynamic processes and do not require the repeated injection of fresh volatile-rich magma or a periodic nucleation of bubbles. Waves of bubbles may also develop in magma chambers and conduits and could play a role in eruption dynamics. ¿ American Geophysical Union 1996