The assessment of completeness of mollusc species lists in selected permanent and temporary floodplain water bodies located within the lower Bug River valley, as well as estimation of the minimum number of samples required to obtain an acceptable efficiency of inventory in individual water bodies, was carried out using sample-based rarefaction curves and non-parametric estimator Chao2. The effect of sampling effort on different measures of species diversity (species richness, Shannon diversity exp(H’)) was examined. Dependence of sampling effort, inventories completeness and diversity measures on habitat stability was analysed by comparing permanent and temporary water bodies. Mollusc assemblages of the investigated water bodies showed high temporal and spatial variability, as well as inter-habitat differences (relatively low Jaccard’s similarity coefficient, J). Significant differences in diversity and composition of mollusc assemblages were found between permanent and temporary habitats, whereas species richness was similar in both permanence groups. In general, both species richness and diversity increased similarly with growing sampling effort. Total richness accuracy reached at least 90% of the predicted value (calculated with Chao2) with 5 to 14 random samples, depending on the water body (10–14 samples in permanent habitats and 5–10 samples in temporary ones).