A seafloor instrument has been constructed to measure a horizontal component of the electric field, using a long antenna (up to 1000m) to increase sensitivity. The instrument has a depth capability of 5200m and may be deployed for several months at a time, collecting over 14 Mbytes of data under the control of a microprocessor. Instrumental noise is mainly caused by the Ag-AgCl electrodes used to make contact with the sea water, and is about 10-²⁴V²/m²Hz at frequencies above 1Hz, following an f^-2 power law at frequencies (f) below 1Hz. For controlled source signals averaging techniques such as synchronous stacking and block averaging have been used to detect narrow band (5.6×10^-4Hz) signals as small as 10^-12V/m. The instrument is designed to include the pressure transducer described by COX et al. (1984) and has already been used for the study of ocean bottom seismic signals, magnetotelluric experiments and controlled source electromagnetic soundings.