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Direct observation by scanning electron microscopy indicates that the presence of bacteria (epimural and unattached) in the hindguts of Crustacea is widespread, occurring across taxa (two infraorders, i.e., Brachyura and Thalassinidae, nine genera, 16 species), feeding types (detritivores, scavengers and carnivores), habitats (mangroves, saltmarshes, sand/mudflats) and continents (North America, South Africa, Australia).Crustacean hindguts clearly represent suitable environments for colonization by micro-organisms, despite the lack of specialized structures or modifications of the gut to facilitate this.Mats of closely-packed epimural rods and scattered epimural rods were the most common types of bacteria observed in the guts of the Crustacea examined, although unattached rods and scattered epimural cocci occurred in some species.There were, however, taxon-dependent differences in colonization characteristics of hindgut bacteria, possibly related to differences in gut lining.Abundance of hindgut microflora was unrelated to the host's taxon, habitat or geographical locality, but appeared to be affected by the feeding habits of the animal.Mats of epimural rods were associated exclusively with detritivores, while cocci were only observed in the hindguts of scavengers and carnivores.Moreover, extensive colonization by epimural rod bacteria (covering > 50% of the hindgut lining) was observed in detritivores only, while carnivores harboured few or no rod bacteria.The detritivore hindgut appears to provide a better environment for microbial habitation than does that of carnivorous crustaceans.In all cases the rod bacteria were monocultures of morphologically identical bacteria and were remarkably similar among crustacean species.The potential significance of prolific microbial colonization in the hindguts of crustaceans deserves consideration.