A film cassette comprises a spool supported within a lighttight shell for rotation to thrust a filmstrip coiled about the spool to the exterior of the shell whether the filmstrip is unexposed or is only partly exposed, and a film exposure status indicator secured to the spool for rotation from an unexposed position to respective partly exposed and fully exposed position for providing visible indications that the filmstrip is only partly exposed or is substantially exposed. The spool has three spaced detents arranged at separate radial locations for respective engagement to lock the indicator in its unexposed, partly exposed and fully exposed positions. The shell has a radial slot in which a locking member is movable between a locking position for individually engaging any one of the detents and a non-locking position for disengaging any one of the detents. Securement means is arranged at the slot for fixing the shell and the locking member releasably to each other only when the locking member is in its locking and non-locking positions.