We review the factors determining lava fountain heights in Hawaiin--stype basaltic eruptions such as those occurring during the recent series of eruptive episodes at the Pu'u 'O'o vent, east rift zone of Kilauea Volcano, Hawaii. Numerical solutions to the equations describing the fluid dynamics of such eruptions predict that lava fountain heights, which are indicators of the velocity of magmas in vents, should be controlled much more strongly by amounts of exsolved volatiles than by any other factors. The next most important factor is the width of the conduit system, which determines frictional losses and can be characterized by the volume flux of magma. The diameter of the surface vent required to accommodate a given sischarge (i.e., instantaneous volume rate of eruption) is also a function of exsolved magma volatile content but is less dependent on this factor than is the fountain height. We simulate fountain heights for typical discharges at Pu'u 'O'o and find implied exsolved gas contents very close to those determined by other methods. The corresponding predicted vent diameters are comparable with (and somewhat smaller than) the observed diameter of the Pu'u 'O'o vent. The discrepancy is probably a consequence of processes occurring in the lava pond which is present over the vent during eruptions. The variation of fountain height with time during eruptive episodes can be used as a measure of changing exsolved magma volatile content, which can be related to processes occurring in the subsurface dike system. ¿ American Geophysical Union 1987