Deadenylation is the first and rate-limiting step in the degradation of many mRNAs in a wide-range of organisms from yeast to higher eukaryotes. It can also play a regulatory role in early development. In this study, we examined the Arabidopsis homolog of poly(A) ribonuclease (PARN), a deadenylase first identified in mammals and absent from yeast. Consistent with the conservation of domains and residues important for catalytic activity, Arabidopsis PARN (AtPARN) expressed in Escherichia coli has poly(A) degradation activity in vitro. Protein localization experiments in plant cells indicate that AtPARN resides in both the nucleus and cytoplasm. To address the importance of the enzyme in vivo, we identified three independent T-DNA insertion mutants of AtPARN which interrupt the gene at different positions between the ATG and the stop codon. All three alleles cause lethality prior to seed germination, indicating that AtPARN is an essential gene first required during early development. Although homologous genes have yet to be inactivated in any other organism, our observations argue for the critical importance of PARN and suggest that it may be essential in many other multicellular eukaryotes.