Interactions between cells and mobile elements such as viruses and plasmids (the mobilome) played a major role in life evolution. In this project we are both studying the mobilome history at the level of an entire domain, Archaea, using genomes already available, and focusing on a particular order of hyperthemophilic archaea, Thermococcales, by the sequencing of 100 closed genomes from Thermococcales isolated in various oceanic hydrothermal sites. Here I will present our results on the study of the diversity and the evolution of the MGEs in Thermococcales. I will present an atypical plasmid family [1], the pT26-2 family, widely spread in the Thermococcales and not only. Bioinformatic analyses suggest that this plasmid family could correspond to a new “plasmidion” [2], as first described recently by Erdmann and co-workers [3]. I will also present the first conjugative plasmid, pT33-3, corresponding to the largest plasmid (103 kb) identified in Thermococcales [4].