We have used a modification of the classical ABA-insensitive screen (Koornneef et al. 1984) to isolate novel mutations in the ABA signal transduction pathway of Arabidopsis thaliana. In our screen, mutants were recovered on the basis of their growth-insensitivity to ABA (GIA) rather than germination-insensitivity. Here we present the isolation of the gia1 mutant as well as the identification of the gia1 gene by positional cloning and complementation studies. GIA1 is predicted to code for a bZIP transcription factor with high homology to previously characterized plant bZIP transcription factors (DPBF1, ABFs and TRAB1) known for their ability to bind ABA-responsive DNA elements. Our results provide in vivo evidence that a bZIP factor may indeed be involved in ABA signaling. Since GIA1 turned out to be identical to ABI5, we designated GIA1 as ABI5 in the present paper.