The shrimp Rimicaris exoculata is considered a primary consumer that dominates the fauna of most Mid-Atlantic Ridge hydrothermal ecosystems. The shrimps harbour in their gill chamber an important ectosymbiotic community of chemoautotrophic bacteria associated with iron oxide deposits. The settlement and development of this ectosymbiosis was investigated using microscopy techniques (light microscopy, LM; and scanning, transmission and environmental scanning electron microscopy: SEM and ESEM, respectively) for shrimps from 2 different vent fields (Rainbow, 36 degrees 14.0' N and TAG, 26 degrees 08.0' N). The results revealed a bacterial re-colonisation after each exuviation and a development of the bacterial community in 5 steps in relation to the moult stages, which were used as a reference time scale. In 287 shrimps from both vent fields, pre-ecdysial stages prevailed in the population, suggesting a short anecdysis and high moulting rate, probably to renew the ectosymbiosis. Comparisons with moult cycles of littoral shrimps suggest that the interval between successive exuviations in R. exoculata may be as short as 10 d. The colours of R. exoculata result from accumulation of iron oxide, which forms a bacteria-associated mineral crust in the gill chambers. The close correspondence between moult stages, the development of the ectosymbiont community and shrimp colours indicate that colour could be used to rapidly determine shrimp moult stages.