16S ribosomal DNA clone library analysis was performed to assess archaeal diversity within three surficial sediment samples obtained from the bathypelagic zone (depth: 2165-3406 m) of the Weddell Sea, Antarctica. The nearly complete 16S rDNA gene (1440 bp) was obtained for 146 clones and 46 phylotypes were defined. The majority of the sequences (99\%) formed three clusters within the Marine Group I Crenarchaeota. The most important cluster, with 78.8\% of the clones, included Candidatus Nitrosopumilus maritimus, a mesophilic archaeon able to oxidize ammonia. The most important subgroup in that cluster was the APA4-0cm subgroup (with 62.3\% of the clones). This subgroup might represent important Crenarchaeota in the functioning of the bathypelagic sedimentary ecosystems of the Weddell Sea because it dominated the clone libraries in all sampling stations, and was found in sediments separated by very large geographic distances. Only one clone grouped within the Euryarchaeota. This euryarchaeal clone could not be affiliated with any of the previously defined clusters and might represent a novel euryarchaeal lineage. © 2007 Elsevier Ltd. All rights reserved.