A new tracer technique for the direct observation of movement and dispersion of estuarine and inshore waters is described. The method utilizes the fluorescent organic pigment, rhodamine B, together with a very stable, compact filter fluorometer. The tracer is very satisfactory with regard to stability, cost, toxicity, and detectability. Its nearly unique fluorescent and absorption spectra minimize the effect of natural background so that concentrations as low as 2 × 10−11have been observed in the field, with expectations that this detection limit can be lowered to 4 × 10−12. Advection and diffusion from approximate point sources discharged in Baltimore Harbor, in Conowingo Lake, and in Chesapeake Bay were measured. Theoretical models of horizontal diffusion are compared to the observed decrease in concentration with time.