An underwater data logger is designed to use a micro-powered recording circuit. The in situ monitor can record up to eight signals from sensors onto an internally-housed, 32,000-word erasable programmable read-only memory (EPROM) unit. The very low energy requirements permit the logger to record hourly measurements for a monitoring period of up to six months without recharging the internal batteries. Equipped with underwater light sensors, the instrument can record photosynthetic photon flux density either through an instantaneous reading or as an integrated value for the period between sampling. In addition to these light measurements, other sensors can be used to simultaneously monitor further parameters such as: temperature, pressure, salinity, currents, etc.. Circuits diagrams depicting unit operation and data processing are described. Computer plots of sample survey data (irradiance between two depths, extinction coefficient, and temperature) are illustrated. This paper further describes such additional features of the underwater logger as the capability of the light sensors, designed for all types of measurements and/or manufactured by different companies, to be evaluated in situ at the same environmental conditions.