The plant hormone ABA (abscisic acid) is able to regulate plant responses to abiotic stresses via regulating the expression of ABA response genes. BIC1 (Blue-light Inhibitor of Cryptochromes 1) and BIC2 have been identified as the inhibitors of plant cryptochrome functions, and are involved in the regulation of plant development and metabolism inArabidopsis. In this study, we report the identification of BIC2 as a regulator of ABA responses inArabidopsis. RT-PCR (Reverse Transcription-Polymerase Chain Reaction) results show that the expression level ofBIC1remained largely unchanged, but that ofBIC2increased significantly in response to ABA treatment. Transfection assays inArabidopsisprotoplasts show that both BIC1 and BIC2 were mainly localized in the nucleus, and were able to activate the expression of the co-transfected reporter gene. Results in seed germination and seedling greening assays show that ABA sensitivity was increased in the transgenic plants overexpressingBIC2, but increased slightly, if any, in the transgenic plants overexpressingBIC1. ABA sensitivity was also increased in thebic2single mutants in seedling greening assays, but no further increase was observed in thebic1 bic2double mutants. On the other hand, in root elongation assays, ABA sensitivity was decreased in the transgenic plants overexpressingBIC2, as well as thebic2single mutants, but no further decrease was observed in thebic1 bic2double mutants. By using qRT-PCR (quantitative RT-PCR), we further examined how BIC2 may regulate ABA responses inArabidopsis, and found that inhibition of ABA on the expression of the ABA receptor genesPYL4(PYR1-Like 4) andPYL5were decreased, but promotion of ABA on the expression of the protein kinase geneSnRK2.6(SNF1-Related Protein Kinases 2.6) was enhanced in both thebic1 bic2double mutants and35S:BIC2overexpression transgenic plants. Taken together, our results suggest that BIC2 regulates ABA responses inArabidopsispossibly by affecting the expression of ABA signaling key regulator genes.