Summary We report here a molecular survey based on 16S rRNA genes of the bacterial diversity found in two deep‐sea vent niches at the Mid‐Atlantic Ridge: hydrothermal sediment (Rainbow site), and microcolonizers made of three different substrates (organic‐rich, iron‐rich and pumice) that were exposed for 15 days to a vent emission. Bacterial diversity in sediment samples was scattered through many bacterial divisions. The most...