Bacteriological analyses of the guts of four common lotic invertebrates are described. The results from these analyses suggest thatSimulium andChironomus digest at least half of the bacteria that they ingest in situ, but no evidence has been found for the digestion of bacteria byBaëtis or byEphemerella. Moreover,Simulium andChironomus do not appear to be selective, with regard to bacterial type, in their digestion. The limitations of the method are discussed and the relative importance of bacteria compared with other components of the insects' diet is assessed. Evidence is presented which supports the hypothesis that bacteria are not as quantitatively important as other components of the detrital food material.

Bodo saltans was isolated from a chalk stream and fed with pure cultures of seven bacteria obtained from the same river. The flagellates were allowed to migrate into suspensions of either of two bacterial species in a T‐maze at 20–22°C. There was a significant difference (P < 0.01) between the numbers of flagellates which migrated into suspensions of different bacteria, which were subsequently arranged in an order of “attractiveness” to the flagellate. Bodo saltans grew successfully in monoxenic suspensions of all seven bacterial strains, but more rapid growth occurred with non‐flagellated than with flagellated bacteria; this may be because while feeding, B. saltans tends to associate with surfaces where non‐flagellated bacteria may also congregate. The efficiency with which B. saltans is able to utilize different bacteria may be influenced by the motility or secretory activities of the bacteria. There was no incontrovertible evidence that B. saltans responds to specific bacterial attractants.

An experimental investigation under field conditions of enclosures containing freshwater pulmonate snails, the macrophyteCeratophyllum demersum and epiphytes, produced evidence of beneficial interactions.Ceratophyllum growth, measured in terms of stem length, numbers of leaf-nodes and growing tips and leaf survival was significantly enhanced in the presence of snails. This effect was attributed to the increased availability of plant nutrients of snail origin, such as phosphates and ammonia, as well as to the snails' action as "cleaning symbionts" in reducing the density of bacterial and algal epiphyton potentially deleterious to macrophytes. Principal component analysis revealed both seasonal and treatment effects of snail grazing on algal epiphyton. Small adnate algal species (e.g.Cocconeis placentula) survived grazing and benefited from the removal of larger, competitor, species. Snail densities increased in all treatments, despite high (86%) juvenile mortality. It is concluded that freshwater pulmonate snails are strong interactors in lentic habitats, enhancing the growth ofCeratophyllum and producing characterisic epiphyte communities. This benefits not only the snails, but also the plants and epiphytes that are associated with them. Thus the interactions between these component parts of the community can be considered as mutualistic.

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