Abstract: An optical probe mounting device for the insertion end of a fluorescence measuring optical probe which facilitates mounting on a biological reactor vessel window fitting. The device consists of a rigid hollow cylinder sized to closely receive the probe and to closely slide within the interior of the fitting. A quartz window seals the insertion end of the cylinder to allow passage of ultraviolet light without attenuation. A flange is sealingly affixed to the cylinder which permits the cylinder to extend a predetermined distance into the vessel. An O-ring extends around the cylinder or a face seal is used on the rear flange of the well, thereby providing a seal upon insertion of the device into the fitting.