Abscisic acid (ABA) insensitive mutants of Arabidopsis thaliana (L.) Heynh. were isolated by selecting plants which grew well on a medium containing 10 μM ABA. From the progeny of approximately 3500 mutagen-treated seeds, five mutants of at least three different loci were isolated. Three mutants were characterized, moreover, by a reduced seed dormancy and by symptoms of withering, indicating disturbed water relations and, therefore, resembled phenotypically the ABA-deficient mutants we described earlier in this species. Two mutants showed in addition only a reduction of seed dormancy. Compared to wild type, all mutants showed similar or increased levels of endogenous ABA in developing seeds and fruits (siliquae). The role of the different genes involved is discussed in relation to the mechanism of ABA action.