The I box is a conserved regulatory motif which is found upstream of plant genes (rbcS, cab and nia) whose transcription is regulated by light and the circadian clock. Gel retardation and UV cross‐linking assays were used to resolve two different groups of I box binding factors (IBFs) in tomato nuclear extracts. Active components of the first group (IBF‐1) recognize the I box of the light‐responsive rbcS promoter; one factor within this group, IBF‐1a, also recognizes the adjacent G box, which has been shown previously to bind a different class of plant transcription factors, the G box binding factors (GBFs). To the limit of experimental resolution, IBF‐1a and GBF compete for the same nucleotides on the G box. Nevertheless, these two activities are biochemically and immunologically distinct. The relative abundance of IBF‐1a shows a vast decrease in dark‐adapted plants. Factors in the second group (IBF‐2), recognize the I box of the nia promoter, which is regulated both by light and the circadian clock; one factor within this group, IBF‐2a, also binds the I box of a second promoter showing similar regulation, the cab promoter. The IBF‐2a binding sites on the cab and nia promoters show extensive homology to a circadian clock‐responsive promoter element from wheat. The abundance of IBF‐2a is diurnally regulated and shows a dramatic induction around the onset of the light period. Transfer of the plants in continuous darkness demonstrates that this induction is under the control of a circadian clock. These data suggest that I box binding factors may be involved in regulation of transcription by light and the circadian clock.