Southeast Asian synchronic fireflies respond to stimulus flashes by phase-shifting their endogenous oscillator. This is called 'flash entrainment'. The releasers for entrainment were studied by changing stimulus flash shape and duration in Pteroptyx malaccae. Stimulus flash shapes and durations were synthesized digitally and delivered by a field-portable computer system. The computer also recorded male firefly flashes that were detected with a photometer. We found that the type of entrainment and its magnitude was influenced by the duration of the entrainment flash and by its shape.