Sea Tech Inc. has developed an in situ fluorometer to measure chlorophyll a fluorescence in aquatic environments. The instrument has been developed within stringent constraints of size, weight and power consumption. The use of custom-designed components, including the flashlamp, excitation and emission filters, and energy storage capacitor for the light source has permitted optimal mechanical, optical and electrical design of the instrument. This new design results in efficient stimulation and detection of chlorophyll a fluorescence. The instrument is not sensitive to ambient light and has excellent stability over time and temperature. Chlorophyll a concentration is measurable on three selectable ranges of approximately 3, 10 or 30 µg/1 full scale with a minimum detectable signal of <0.02 gg/1. Time constants of 0.1, 1, 3 and 10 seconds are selectable to smooth the output data. Power requirements are nominally 12 VDC at 150 mA, and output signal is 0 to 5 VDC. These power requirements and signal levels make the fluorometer compatible with most oceanographic moored and profiling data acquisition systems. Operating depth for the instrument is rated at 500 meters with a plastic housing or 3000 meters with a stainless steel pressure housing.