1. The gastropods Segmentina nitida, Anisus vorticulus and Valvata macrostoma occur in drainage ditches on grazing marshes that are now among the most threatened wetland systems in western Europe. Although each of these species is listed in the UK Red Data Book (RDB) and Biodiversity Action Plan, influences on their distribution are poorly understood. 2. To improve management information, the within-channel distributions of the three snails were examined in 20 ditches in southeast England. Abundance, vegetation cover and other environmental factors were recorded across the ditch profile near the surface (<0.2 m) and at depth (<0.15 m from the benthos). 3. All three species were significantly more abundant near the surface than at depth. Below 0.6 m, ditches had significantly reduced concentrations of dissolved oxygen (<0.6 mg L-1, < 5% saturation), possibly sufficient to limit the occurrence of gastropods dependent wholly or partly on aqueous gas exchange. 4. There were no other systematic variations in abundance between the ditch margins and centre channel. However, across all ditches and samples in the survey, each species' abundance varied significantly with vegetation structure. S. nitida was most numerous where there was least open water and hence most vegetation, V. macrostoma among emergent stands and A. vorticulus in ditches with floating vegetation but few submerged plants. 5. These microdistributional data support large-scale surveys in illustrating the potential importance of vegetation management for these snails; providing that other requirements are satisfied, ditch management could favour each RDB species by optimizing particular vegetation features. Apparent preferences for vegetation structure rather than particular channel locations suggest that refuges left during ditch clearance could be located anywhere in the ditch. Factors that reduce oxygen concentrations below 1 mg L-1, such as eutrophication, might be detrimental.