Twenty on-bottom gravity stations were taken with a LaCoste-Romberg model G gravity meter inside the submersible Alvin, over a 6 km long cross-strike profile which straddles the East Pacific Rise spreading center at 21 degrees N. The residual Bouguer anomaly shows a 1.5 mGal low with a total half-width of 1.7 km centered about the spreading axis and hydrothermal vents. Uncertainties in the ocean depth are believed to contribute a standard error of about 0.3 to 0.4 mGal for the gravity data. The observed elevation factor of 0.1552 mGal/m indicates that the upper ocean crust bulk density is 2.62 + or - 0.09 g/cm 3 . On the other hand, 90 rock samples give a density of 2.90 + or - 0.01 g/cm 3 . This suggests that the large-scale porosity of the upper crust is 14 percent. The gravity anomaly was inverted to find the ideal body with the smallest maximum density contrast which satisfies the data and their associated errors. This body extends from the sea floor to 2 500 m depth and has a density contrast of -0.043 g/cm 3 . If the density contrast is allowed to be as large as -0.1 g/cm 3 , the top of the ideal body can be as deep as 300 m and its bottom as deep as 1 300 m; alternatively with the body top at the sea floor, the bottom of the body can be no shallower than 500 m. The ideal body is, therefore, restricted to the dikes and flows of the upper ocean crust. The body is not thought to represent magma in the shallow crust, but rather fractured crust filled with heated seawater.