The Rainbow hydrothermal plume was discovered during a recent geophysical survey along 200km of the Mid-Atlantic Ridge (MAR), SW of the Azores Triple Junction, in which at least seven new sites of hydrothermal activity were identified. Here, we present the first hydrographic study of the Rainbow plume 36¿ 15'N, the strongest of the features located during that survey. The plume is detectable from real-time in situ nephelometer anomalies and extends 10-15km, W-E in a non-transform discontinuity (NTD) between two adjacent ridge-segments. Maximum anomalies in the Rainbow plume indicate particle enrichments at least as large as those seen directly above the Trans Atlantic Geotraverse (TAG) vent-field (26 ¿N, MAR). Analysis of hydrographic data indicates a vent source at Rainbow with a thermal output of up to 98 MW, representing 140% of the thermal output previously attributed to TAG. Both lines of evidence indicate the Rainbow plume to be the strongest such feature yet found on the MAR. ¿ American Geophysical Union 1996