AbstractIn the microbenthos of the Czarna Staszowska River (south-eastern Poland), the diatom Geissleria declivis (Hustedt) Lange-Bertalot & Metzeltin, a species rarely noted in Poland, but also in Europe and the world, was recorded. In this study, the taxonomic, morphological and ecological analysis of the diatom species is presented. Also, the specific geological structure of the region of Poland, and physico-chemical water parameters of the river where the species was now observed, are described. The water of that section of the river where G. declivis occurred is included in the oligosaprobic to beta-mesosaprobic zone (Water Quality Class I-II). Limestone and gypsum are the main components of the geological structure of the river valley. Accompanying species of G. declivis were: Achnanthes pusilla (Grun.) De Toni, Amphora copulata (Kütz.) Schoem. & Arch., Fragilaria brevistriata Grun. in Van Heurck, Fragilaria pinnata Ehr., Geissleria decussis (Hust.) Lange-B. & Metz., Hippodonta capitata (Ehr.) Lange-B., Metz. & Witk., Hippodonta costulata (Grun.) Lange-B., Metz. & Witk., Navicula concentrica Carter, Navicula cryptocephala Kütz., Navicula novaesiberica Lange-B., Navicula rostellata Kütz., Nitzschia archibaldii Lange-B., Parlibellus protractoides (Hust.) Witk., Lange-B. & Metz. Taxa that are indicators of very good water quality were: Achnanthes peragalli Brun & Heribaud, Achnanthes subatomoides (Hust.) Lange-B. & Arch., Achnanthidium kranzii (Lange-B.) Round & Bukht., Cymbopleura hybrida (Grun.) Kram., Navicula densilineolata (Lange-B.) Lange-B., Navicula notha Wallace, Nitzschia fibulafissa Lange-B. Taxa preferring waters rich in calcium and chalk were: Aneumastus stroesei (Ostrup) Mann & Stickle, Diploneis parma Cleve, Hippodonta lueneburgensis (Grun.) Lange-B., Metz. & Witk., Navicula gottlandica Grun. in Van Heurck, Navicula jakovljevicii Hust., Navicula moskalii Metz., Witk. & Lange-B., Sellaphora mutatoides Lange-B. & Metz., Sellaphora pseudopupula (Krasske) Lange-B. Navicula oppugnata Hust.