A fluid pulse generator for use in a drill string comprises an elongate obstruction member mounted in a fluid passage for driven pivoting about a pivot axis transverse to the fluid passage, obstruction of the fluid passage by the obstruction member being variable in relation to pivotal position of the obstruction member. Telemetry signals can be transmitted along the drill string by driven pivoting of the obstruction member, to generate data pulses in drilling fluid in the drill string. Pressure-locking of the obstruction member in a maximally obstructive position can be counteracted by provision of a bypass arrangement to allow bypass flow at a leading end of the obstruction member.