The Arabidopsis ABI1 and ABI2 genes encode two protein serine/threonine phosphatases 2C (PP2C). These genes have been originally identified by the dominant mutations abi1-1 and abi2-1, which reduce the plant's responsiveness to the hormone abscisic acid (ABA). However, recessive mutants of ABI1 were recently shown to be supersensitive to ABA, which demonstrated that the ABI1 phosphatase is a negative regulator of ABA signalling. We report here the isolation and characterisation of the first reduction-of-function allele of ABI2, abi2-1R1. The in vitro phosphatase activity of the abi2-1R1 protein is approximately 100-fold lower than that of the wild-type ABI2 protein. Abi2-1R1 plants displayed a wild-type ABA sensitivity. However, doubly mutant plants combining the abi2-1R1 allele and a loss-of-function allele at the ABI1 locus were more responsive to ABA than each of the parental single mutants. These data indicate that the wild-type ABI2 phosphatase is a negative regulator of ABA signalling, and that the ABI1 and ABI2 phosphatases have overlapping roles in controlling ABA action. Measurements of PP2C activity in plant extracts showed that the phosphatase activity of ABI1 and ABI2 increases in response to ABA. These results suggest that ABI1 and ABI2 act in a negative feedback regulatory loop of the ABA signalling pathway.