The tidal exchange between a bay and the ocean often results from a process termed tidal pumping caused by an asymmetry between the flow during the ebb and flood tides. For many bays the ebb flow is concentrated into a jet which occupies the region just offshore from the mouth of the bay, while the flood flow is drawn in rather radially from the region all around the mouth. A significant variation in tidal exchange with tidal range is based on the notion that the length scale of the ebb jet increases more rapidly than the length scale of the flood withdrawal zone as the tidal range increases. Exchange between San Diego Bay and the Pacific Ocean was quantified using a combination of flow measurements by acoustic Doppler current profiling and tracer measurements using a naturally occurring ultraviolet fluorescence tracer. We found that variations in exchange with tidal range could be isolated by separately evaluating the ebb and flood tidal transport budgets. The tracer transport during the ebb increased rapidly with tidal range, while during the flood tide, the transport increased more gradually. The resulting difference in tidal transport between the ebb and flood accounts for the exchange between the bay and ocean. For weak tides, the exchange tends to increase rapidly with increasing tidal range, while for stronger tides, the exchange is more constant. An empirical equation for the tidal exchange developed from the measurements is consistent with theoretical tidal pumping models and supports the concept that the exchange at the mouth of San Diego Bay is dominated by tidal pumping. ¿ 1999 American Geophysical Union