RNA degradation plays an important role in the control of gene expression in all domains of life, including Archaea. While analyzing RNA degradation in different archaea, we faced an interesting situation. The members of a group of methanogenic archaea, including Methanocaldococcus jannaschii, contain neither the archaeal exosome nor RNase II/R homologs. However, looking for potential ribonucleases revealed proteins related to the recently discovered ribonuclease RNase J. RNase J is unique among known ribonucleases because it may combine endo- and 5'→3' exo-ribonucleolytic activities in a single polypeptide. Here, we report the characterization of the ribonuclease activities of three RNase J homologs encoded in the genome of the methanogenic archaeon Methanocaldococcus jannaschii. The analysis of the recombinant archaeal proteins purified from E. coli revealed an optimal activity at 60°C. Whereas mjRNase J1 and -J3 displayed exclusively 5'→3' exonucleolytic activity, mjRNase J2 is an endonuclease with no apparent exonuclease activity. The exonucleolytic activity of both mjRNase J1 and -J3 is enhanced in molecules harboring monophosphate at the 5' end. mjRNase J3, and to some extent mjRNase J2, degrade ssDNA. Together, these results reveal that in archaea lacking the exosome and RNase II/R, RNA and perhaps also DNA are possibly degraded by the coordinated activities of several RNase J proteins. Unlike bacteria, in archaea RNase J proteins provide separately the exo- and endonucleolytic activities that are probably essential for RNA degradation.