The LaCoste and Romberg surface-ship gravity meter S-9 has been used to measure gravity at sea on the research vessels Hidalgo, Vema, and Aragonese. Tests were made to determine reliabilities of gravity recordings with underdamped and overdamped horizontal accelerometers in the meter system. The tests included hovering over a bottom gravity meter, making measurements over areas of known gravity, making measurements at different ship speeds and headings with respect to the waves past offshore petroleum platforms where gravity is known, comparing gravity at intersections of profiles and along reversed profiles, and making measurements while inducing sinusoidal (fishtailing) and saw-tooth (yaw) ship motions. It was observed that (1) the meter drift rate was less than 1 mgal in several months; (2) successive measurements in harbors could be repeated within 1 mgal; (3) the calibration factor supplied by the manufacturer was quite accurate; (4) the reliability while hovering (not drifting) in even moderately rough seas was within 4 mgal; (5) the meter readings were accurate at very low Browne corrections, but under normal cruising conditions, when moderate to large Browne corrections were indicated by the horizontal accelerometers, appreciable errors in gravity recordings were obtained, primarily due to long-period horizontal motions to which the particular accelerometers used did not respond completely. An empirical curve relating error in gravity reading to measured Browne correction was obtained. This curve has been used to correct gravity recorded on the Hidalgo; recordings thus corrected were reliable to within approximately ±10 mgal. The horizontal accelerometer characteristics have been modified since these tests.