We report the identification and cloning of an allantoate amidohydrolase (allantoate deiminase, EC 126.96.36.199) cDNA from Arabidopsis thaliana (L.) Heynh. This sequence, which we term Arabidopsis thaliana Allantoate Amidohydrolase (AtAAH), was shown to be functional by complementation of Saccharomyces cerevisiae dal2 mutants, blocked in allantoate degradation. Following transfer to a medium containing allantoin as the sole nitrogen source, Ataah T-DNA insertion mutants were severely impaired and eventually died. Ataah mutants demonstrated higher allantoate levels than wild-type plants in the presence and absence of exogenous ureides, supporting a block in allantoate catabolism. AtAAH transcript was detected in all tissues examined by RT-PCR, consistent with a function in purine turnover in Arabidopsis. To our knowledge this is the first allantoate amidohydrolase gene identified in any plant species.