Moored current measurements made from April 1994 to June 1995 reveal the vertical and temporal structure of the vorticity over the northern segment of the Juan de Fuca Ridge. At a depth of 2100m, near the thermal and particle maximum associated with the neutrally buoyant hydrothermal plume, the relative vorticity is anticyclonic with a mean value of nearly -0.09f, with variability of the same magnitude. This arises from cross-ridge shear in the horizontal currents at this level. In the vertical, the anticyclonic shear has a peak value at plume level decreasing in magnitude both upward and downward from depths of 2100--2150m. As the bottom is approached, the transverse shear changes sign with cyclonic values of up to 0.09f below 2250m depth. This vorticity signature is suggestive of dynamics of a line source of buoyant fluid entering the plume at its neutral density horizon and not due to individual point sources. ¿ 1998 American Geophysical Union