A two-stage, fully self-retracting, telescopic fluid actuator suitable for high speed ejection applications comprises a detent 38, 64, 26, which locks a cylinder 6 in an extended position for extension of a piston 3 therefrom. The detent is released by retraction of the piston into the cylinder again, e.g. under spring bias, so that the actuator is fully self-retracting. A locking ball 38 of the detent also serves to latch the piston and cylinder to each other for the first stage of the actuator extension. In an alternative embodiment, cam means (80, 117 FIGS. 10, 11 and 13) and plungers (90, FIGS. 10, 12, 13 and 15) perform the detent and latching functions.