Light affects plant growth and development throughout the life cycle. However, light signals do not function autonomously but should be integrated with endogenous developmental factors such as the plant hormone auxin to specify correct developmental decisions. We have previously reported that the Arabidopsis shy2-1D mutation alters various light responses, including highlyphotomorphogenic development in darkness. Here we show that the mutation also alters various auxin responses, including constitutive formation oflateral roots and reduced auxin sensitivity in inhibition of hypocotyl and root growth. The mutation is a gain of function mutation occuring in the IAA3 gene, one of the Aux/IAA family genes encoding putative transcription factors of auxin-responsive genes. These results suggest that IAA3/SHY2 may play important roles in both light- and auxin-mediated development. Considering that Aux/IAA proteins and auxin response transcription factors interact with one another, we propose that IAA3/SHY2 may integrate light signals into auxin-mediated developmental responses.