Synchronous flashing occurs in certain species of Southeast Asian and North American fireflies. Most Southeast Asian synchrony involves stationary congregating fireflies, but North American synchrony occurs in flying fireflies that do not congregate. Southeast Asian synchrony is usually continuous, but North American synchrony is interrupted. Photuris frontalis, the only member of the North American genus Photuris to synchronize, shows an intermittent synchrony. This involves synchronization and repeated re-synchronizations while in flight. The precision that occurs at the start of synchrony was studied in Ph. frontalis using caged fireflies and photometry. Barrier experiments (using two fireflies) or flash entrainment experiments (using one LED and one firefly) were performed to measure the temporal precision of the first entrained flash. In both cases, the first entrained flash was close to unison synchrony (phase = 1.0) and showed little variability. The behavioral implications of the ability to synchronize with the first entrained flash are not known, but it might facilitate male-male interactions during brief, transient encounters such as maintaining distance between closely flying males in search of females.