To understand how light entrains circadian clocks, we examined the effects of light on a gene known to encode a state variable of a circadian oscillator, the frequency (frq) gene. frq is rapidly induced by short pulses of visible light; clock resetting is correlated with frq induction and is blocked by drugs that block the synthesis of protein or translatable RNA. The speed and magnitude of frq induction suggest that this may be the initial clock-specific event in light resetting. Light induction overcomes frq negative autoregulation so that frq expression can remain high in constant light. These data explain how a simple unidirectional signal (light and the induction of frq) may be turned into a bidirectional clock response (time of day-specific advances and delays). This light entrainment model is easily generalized and may be the common mechanism by which the intracellular feedback cycles that comprise circadian clocks are brought into synchrony with external cycles in the real world.