The relative importance of expression and symbolic content in acoustic communication of cetaceans has been debated. These topics may be examined by observing how general rates of vocalizations change with excitatory state and how specific vocalizations vary independently of state and depend upon content. In this study, whistles (0.32 to 22 kHz) of two captive bottlenose dolphins (Tursiops truncatus) were recorded underwater before, during, and after six swim sessions in which the behavior of the dolphins and four human swimmers was controlled by a trainer on the dock. Sonograms of 205 min of recorded whistles were analyzed. Variation of whistle-type and rate was observed relative to the three contexts. The contours of distinguishable whistles (n=591) revealed 32 types. Ten types were observed during one context only; during the swim sessions. Almost half of the whistle types (15 to 32) were more likely to occur prior to swim sessions and the others were more likely during swim sessions. None were more frequent after swims. Rate of whistles was significantly higher than expected during the swim sessions (z=7.21) and lower than expected after the swim sessions (z=-0.03).