The dynamic signatures of two megaplumes above the Juan de Fuca Ridge are analyzed. The chemical properties of these two lenslike masses of water were described by Baker et al. (1989) and clearly indicate that they were generated by massive and rapid ventings of hot hydrothermal fluid from the ridge. Both are nearly circular with radii of about 6.5 km. The isopycnals bow upward around these cores of anomalous water, leading to an anticyclonic circulation. A cyclogeostrophic balance gives maximum currents at the edge of the core of 0.11 m s-1 for the first megaplume (MP1) and 0.07 m s-1 for the second megaplume (MP2). Currents extend beyond the core to a radius of 12--15 km. The centers of the cores are in nearly solid body rotation with relative vorticity of -0.5f (MP1) and -0.3f (MP2) and potential vorticity anomalies, expressed in units of equivalent relative vorticity, of -0.8f (MP1) and -0.6f (MP2), where f is the Coriolis frequency. The aspect ratio of each megaplume gives a Burger number of 0.22. In terms of these nondimensional numbers, the megaplumes are very similar to eddies of Mediterranean water found in the eastern Atlantic (meddies), despite their very different origin. ¿ American Geophysical Union 1994