The evolution of the cellular structure of a gas-detonation wave propagating in a circular pipe was investigated. The propagation regimes, hereafter ‘detonation modes,’ for which the organization of the flow in the detonation-wave front and, accordingly, the shape of the cellular structure have the best symmetry and recurrence, have been revealed. It is shown that the realization of only the single-mode regime of propagation is improbable and that the complete state of the wave-front structure as a function of time can be represented as a superposition of periodic functions corresponding to the states of neighboring detonation modes, and their normalized amplitude is proportional to the probability of the presence of the state of a given mode when a gas-detonation wave propagates.