An improved method of detecting buoyant hydrothermal plumes and locating their source vents is introduced. Plumes are detected by computing fluid stability from conductivity, temperature, and depth measurements acquired during navigated, towed, vertically oscillating casts over the Endeavour Segment of the Juan de Fuca Ridge. For each instability detected, the maximum range to its hydrothermal source is estimated by multiplying a theoretical plume equilibration time by a measured current velocity. Using an estimate of current direction, the method reliably locates plume sources where they are known to exist: in all of the focused vent fields mapped by submersible and in several isolated, diffuse flow sites. The method generates a distribution of hydrothermal sources that is more consistent with variations in surface permeability than with circulation cells spaced evenly along a uniformly permeable axis. Axial instabilities are nearly continuous along the heavily fissured and fractured western wall of the axial valley. Beyond the axial valley, instabilities evidence that hydrothermal upflow penetrates the outer slopes of the ridge crest, probably along a boundary between sheet and pillow flows. ¿ 1999 American Geophysical Union