A fast-spreading segment (12 cm/yr (super -1) ) of the East Pacific Rise near 12 degrees 50' N, explored in detail by surface ship and manned submersible, displays intense hydrothermal activity. The rise crest (about 1,500 m in width) with a regional depth of 2,600 m is occupied at its center by an axial graben (<500 m in width). Off-axis seamounts (6 km in basal diameter; <400 m in height) located at less than 7 km from the axis have obliterated the linear fabric of the accreting plate boundary region.The central graben where the freshest volcanic flows are found is the site of the hydrothermal activity. More than 80 active and inactive hydrothermal deposits were recognized, forming localized and discontinuous fields averaging about 50 m in diameter, scattered on a narrow band (less than 200 m in width) along a 20-km segment of the axial graben. Each field consists of irregular and conical-shaped edifices varying from about 1 up to 25 m in height (average of 5-6 m in height and 1-2 m in diameter); they are made up of Fe, Cu, and Zn sulfides. It is estimated that the amount of sulfide formed in the past 100 years (Hekinian et al., 1983b) in the area of the explored axial graben does not exceed 20,000 metric tons. The most extensive hydrothermal deposits (about 800 m long, 200 m wide, with a volume about ten times higher than that of the axial graben) were found on the summit and the flank of an off-axis volcano, herein termed the southeastern seamount. This hydrothermal material consists mainly of ocherous Fe hydroxide (goethite) comprising about 62 percent of the bulk product recovered from the southeastern seamount; it overlies Fe-rich massive sulfide (24%), silica-rich sulfide (13%), and massive Fe-Cu sulfides (1%). Localized patches of cobaltiferous pyrite were detected in silica-rich phases replacing sulfides and in massive Fe sulfides.The volcanics that erupted in the axial graben and are associated with the hydrothermal deposits are generally less fractionated (i.e., more primitive) basalts than those found on the adjacent southeastern seamount. They are olivine-plagioclase tholeiites with low TiO 2 (<1.2%), low alkali (K 2 O < 0.15%), and a low FeO T /MgO ratio (>1.3%); titania-alkali-enriched olivine (Ti-K) basalts with high TiO 2 (>1.4%) and high alkali content (Na 2 O > 3.0%; K 2 O > 0.3%) occur on the southeastern seamount. The fractionated tholeiites are believed to have been derived from primitive melts during differentiation which took place within a magma reservoir--asymmetrical with respect to the axis of the graben--which supplied both axial and off-axial lava flows. The Ti-K-enriched basalts are believed to have been produced from an alkali-enriched magmatic source.