The Cohassett flow of the Grande Ronde Basalt contains two vesicular zones separated by 24 m of dense, sparsely vesicular basalt. The vesicular zones represent accumulations of upward-migrating bubbles sequentially frozen-in by the downward passage of the upper solidification front. The separation of the two zones suggests a hiatus in the exsolution of aqueous vapor from the lava. The flow-top vesicular zone formed during and immediately following emplacement of the flow from vapor bubbles nucleated during ascent and eruption, whereas the internal vesicular zone formed after emplacement from bubbles apparently nucleated at the lower solidification front. Homogeneous nucleation of vapor was probably initially driven by large oversaturation pressures in the erupting lava. After emplacement, heterogeneous nucleation of vapor was probably initiated by crystallization in the margins of the flow.