To investigate the undulating median fin propulsion and its potential for implementation in man-made underwater vehicles, a "fin actuator" has been developed. The device consists of eight parallel bellows actuators or PBAs (fin rays) arranged in a series and interconnected via a flexible material (fin membrane). The PBAs are pneumatically driven and allow for bending movements in any orientation plane. Position data are provided by flexible electrogoniometers. The forces generated from the fin actuator undulations are measured by a pair of tri-axial force sensors. Initial testing has concentrated on 2D fin ray motions, whereby each PBA moves laterally, with the deflection angle performing a sinusoidal variation. Preliminary measurements, for a range of the propulsive waveform parameters, demonstrate reversible thrust generation and good agreement with the theoretical predictions of a momentum-based model.