Exonucleolytic degradation of the poly (A) tail is often the first step in the decay of eukaryotic mRNAs and is also used to silence certain maternal mRNAs translationally during oocyte maturation and early embryonic development. We previously described the purification of a poly (A)‐specific 3′‐exoribonuclease (deadenylating nuclease, DAN) from mammalian tissue. Here, the isolation and functional characterization of cDNA clones encoding human DAN is reported. Recombinant DAN overexpressed in Escherichia coli has properties similar to those of the authentic protein. The amino acid sequence of DAN shows homology to the RNase D family of 3′‐exonucleases. DAN appears to be localized in both the nucleus and the cytoplasm. It is not stably associated with polysomes or ribosomal subunits. Xenopus oocytes contain nuclear and cytoplasmic DAN isoforms, both of which are closely related to the human DAN. Anti‐DAN antibody microinjected into oocytes inhibits default deadenylation during progesterone‐induced maturation. Ectopic expression of human DAN in enucleated oocytes rescues maturation‐specific deadenylation, indicating that amphibian and mammalian DANs are functionally equivalent.