The conditions of pumice generation in Plinian eruptions are studied. A physical model describes the behavior of gas bubbles in a magma fragment which is carried upward in a volcanic conduit and an atmospheric eruption column. The effects of pressure release and cooling are calculated for a range of eruption conditions. The magma fragment expands in the conduit and stops expanding soon after leaving the vent, when a thin viscous rind forms against the cold mixture of magmatic gas and air. This rind prevents further volume changes. Pumice vesicularity is a function of the decompression rate in the conduit, which depends on ascent velocity and fragmentation depth. It is also sensitive to the cooling rate in the atmospheric column, which depends on vent radius and mass discharge rate. Different fragments follow different trajectories in the column and are subjected to different cooling rates. This generates a range of vesicularities which reflects the eruptive conditions. All else being equal, pumice vesicularity increases as magma viscosity decreases. These predictions are consistent with observations. Pumices provide quantitative constraints on conduit flow conditions and mass discharge rate. These concepts are applied to two Plinian eruptions. Vesicularity values for the Bishop Tuff, Long Valley caldera, require a mass discharge rate between 108 and 109 kg s-1. Vesicularity variations during Plinian phase 1 of the Minoan eruption, Santorini, are explained by the conduit radius increasing from about 30 m to 120 m. Both cases require large decompression rates in the eruption conduit, suggesting that flow pressures were close to lithostatic values.¿ 1997 American Geophysical Union