Hydrofast Cruise of the R.V. Jean Charcot addressed the important problem of the variation of hydrothermal activity along the strike of the Ridge over a distance of 100 km with respect to the structure of the ridge axis. This cruise was conducted along the East Pacific Rise between the OSC at 11 degree 45'N and the topographic high at 13 degree N. To reach that objective, a new tool, the dynamic hydrocast, was used. This technique obtains average samples mile per mile, horizontally, at 4 different depths above the bottom. Temperature is recorded at the same time. In addition, the side-scan sonar SAR gave a continuous picture of the fissures along the strike and of the different facies within the 300 to 400 m width of the axial graben. A magnetometer and a thermistance array towed with the SAR completed the set of data.