Swimming-type robotic fish are being developed as mobile sensor platforms that have the potential to outperform existing underwater vehicles in terms of manoeuvrability, miniaturisation and silent operation. Conducting polymer actuators are potential candidates for propelling a flapping tail fin for a robotic fish. This study introduces a method for directly measuring the thrust force generated by a polypyrrole (PPy) powered tail fin. The effects of voltage stimulus (waveform shape and frequency) and tail fin shape on the thrust force were determined. A square wave voltage input was shown to generate the highest thrust forces as fast bending actuation was induced. The thrust force tended to decrease at higher operating frequencies as a result of the reduced actuation occurring in the PPy layers. Increases in the tail fin area tended to lower the frequency where the peak in thrust force was achieved. Tail fin shape was also shown to be important to the force-frequency behaviour. When attached to an untethered robotic fish, the swimming speed tended to increase with increasing voltage input frequency. There was also an optimum frequency that enabled the fastest acceleration from rest that happened to be higher than the frequency that gave the peak thrust force. This was presumed to be due to the complicated swimming behaviour involving the side-to-side movement of the fish nose at low flap frequencies. The study provides valuable insight into the means for tailoring polymer actuator performance to achieve maximum fish swimming speeds. Scheme 1. Structure of polypyrrole and its electrochemical oxidation and reduction.