Photoperiodic responses, such as the daylength-dependent control of reproductive development, are associated with a circadian biological clock. The photoperiod-insensitive early-flowering 3 (elf3) mutant of Arabidopsis thaliana lacks rhythmicity in two distinct circadian-regulated processes. This defect was apparent only when plants were assayed under constant light conditions. elf3 mutants retain rhythmicity in constant dark and anticipate light/dark transitions under most light/dark regimes. The conditional arrhythmic phenotype suggests that the circadian pacemaker is intact in darkness in elf3 mutant plants, but the transduction of light signals to the circadian clock is impaired.