The demise of Britain's only native crayfish (Austropotamobius pallipes (Lereboullet)) has prompted conservation-led translocations to safe isolated stillwaters (Ark sites). Many translocations represent introductions rather than re-stocking; hence it is important to ensure that there will not be a detrimental impact on communities already present. Current knowledge is insufficient to predict likely effects, although gut contents analyses suggest that A. pallipes is omnivorous and exhibits ontogenic diet shifts. A mesocosm study was conducted with the aim to first, determine community impacts of introducing A. pallipes on the benthic invertebrate community, and second investigate if impacts vary with crayfish life-stage. All crayfish life-stages strongly reduced shredder and grazer abundance, particularly thin-shelled Lymnaea snails. The small snail Physa fontinalis was preferentially consumed by juveniles, perhaps reflecting different prey-handling ability. Adults showed greater reliance on terrestrially derived detritus compared to juveniles. There were limited effects on other trophic levels, although by reducing shredders that process basal resources, crayfish impacts may be felt through the community in the long-term.