Diversity-generating retroelements (DGRs) facilitate the rapid diversification of DNA sequences through a process involving reverse transcription and retrohoming, which generates multiple variants of a target gene. To date, DGRs have only been identified in bacteria and bacteriophages, but Paul et al. now report the discovery of archaeal DGRs. Analysis of viral metagenomes obtained from methane seep sediments revealed a putative archaeal virus that encodes a complete and active DGR, the target gene of which shared structural homology with a tail fibre protein, suggesting that the DGR might have a role in generating alternative tail fibres to possibly expand host tropism. The authors also identified multiple DGRs in the genomes of two uncultivated subterranean Nanoarchaeota, which suggests that DGRs might be prevalent in archaea and might confer selective advantages in subsurface environments.