An experiment to measure the dispersion of a passive tracer injected at 300 and 1000 m depth in the Pacific 600 miles southwest of San Diego is described. Rhodamine dye was released as a (very nearly). point source, and the subsequent growth of the patch was mapped at various times, up to 66.2 hours after release, using a self‐propelled underwater research vehicle (SPURV), which carried temperature, conductivity, pressure, and dye sensors on a depth cycling trajectory through the patch. The analysis shows that the horizontal diffusion rates decrease with depth, that it is difficult to resolve the mixing in terms of a constant diffusion parameter based on any one of a number of models, that the 300 m results agree with those obtained by Schuert (1970), and that the patch growth began to develop eddy behavior at 66.2 hours when the rms second moment of the dye distribution was (r2)l/2 = 210 m. For the purpose of predicting horizontal mixing, the results indicate that eddy diffusivities of 1150 and 360 cm2/s at 300 and 1000 m, respectively, are the best fit values for spatial scales <200 m horizontally. Although we have focused on horizontal (isopycnal) diffusion, a few aspects of the vertical (diapycnal) characteristics of the patch are also discussed.

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