Water-borne, fluorescent tracers are conventionally detected by fluorometrically analysing liquid samples taken discretely or continuously from the water body under test. A continuous, automatic, film system described here detects tracer concentrations of one part per billion (1 in 109), provides a time distribution of concentration changes, and combines low cost with ready portability. Field installations are self-operative for up to ten days. A single laboratory-based fluorometer can easily analyse the concurrent data input from fifteen or more field installations. Data can also be stored prior to fluorometry. Fluorescent tracers have been detected successfully by the film system under experimental conditions to gauge streamflow, to produce flow hydrographs, and to determine the travel time of streamflow. Further refinements should be made by potential users to optimize the system for specific needs.