Abstract: A submersible fluorometer of the type having a source transmitting light pulses into the fluid in which the fluorometer is submersed to cause material in suspension to fluoresce and a fluoroescence detector, wherein a second detector is used to measure the intensity of light output of the source during each pulse of light, the outputs of the fluorescence detector and the second detector are sampled during each pulse of light and a ratioing circuit is used to determine the intensity of the fluorescence relative to the intensity of the light causing fluorescence. This arrangement compensates the output for both short and long term drift effects particularly in the light source. Preferably both the light transmitted to the said fluid and the light received therefrom is filtered, in the first case to remove light having wavelengths equal to or longer than that of the fluorescence and in the second case to remove light having wavelengths shorter than that of the fluoroscence.