We report the first observations based on acoustic imaging of large-scale structure and time variability of buoyant plumes emanating from black smoker-type seafloor hot springs. Three-dimensional plume reconstructions were made from a digital data set of acoustic backscattering information recorded on a prototype submersible-mounted sonar system. The acoustic images of two adjacent black smokers depict volume and show zones of flow organization (meters to tens of meters) in the lower 40 m of the buoyant plumes. The two plumes coalesce, bend in the prevailing current, exhibit short-term (minutes) variation in cross section, and rapid (seconds) turbulent eddy variations at small scales (<1 m). The plume imaging system is being developed for determination of plume dynamics, flux determinations when combined with chemical and thermal measurements, and long-term monitoring of the activity of seafloor hydrothermal fields.