This paper presents the results of the Bacillariophyta species diversity study in water bodies of the Poltava-plain algofloristic region (PPAR). Samples were collected according to the conventional technique and cleaned using concentrated H2O2. Diatoms were investigated by means of light and scanning electron microscopy. As a result of the study, 601 species (629 infraspecific taxa, ist) of diatoms were revealed. They belong to 4 classes, 17 orders, 38 families and 87 genera. Among them 195 taxa are first cited for the flora of Ukraine. The data obtained increased by 58.6% the list of taxa known to PPAR. The total number of species found in the studied region (taking into account the literature data) is 705 (756 ist). It compriced 58.7% of the diatoms found in the continental reservoirs of Ukraine. Class Bacillariophyceae (87.4% of the total) leads in species diversity, Fragilariophyceae (8.8%), Mediophyceae (2.5%) and Coscinodiscophyceae (1.3%) were much poorer. Among families, Gomphonemataceae, Bacillariaceae, Cymbellaceae, Naviculaceae, Pinnulariaceae, Stauroneidaceae, Surirellaceae, Sellaphoraceae, Catenulaceae, Achnanthidiaceae, Fragilariaceae, Staurosiraceae, Diadesmidaceae, Eunotiaceae and Stephanodiscaceae were the richest in species. The group of leading genera include Gomphonema (C.Agardh) Ehrenberg, Navicula Bory, Nitzschia Hassall, Pinnularia Ehrenberg, Sellaphora Mereschkowsky, Amphora Ehrenberg, Caloneis Cleve, Cymbella C.Agardh, Stauroneis Ehrenberg, Placoneis Mereschkowsky, Planothidium Round & Bukhtiyarova, Eunotia Ehrenberg, Luticola D.G.Mann, Tryblionella W.Smith, Surirella Turpin, Fragilaria Lyngbye, Encyonema Kützing, Diploneis (Ehrenberg) Cleve, Hipodonta Lange-Bertalot, Metzeltin & Witkowski and Epithemia Brébisson. The ratio of the number of diatom taxa in studied water bodies of PPAR is 1 : 2.6 : 17.6. The distribution of the identified species diversity between different types of water bodies was uneven ranging from 583 species (611 ist) in rivers to 169 species in natural ponds. Comparison of the lists of species of different types of PPAR reservoirs using the coefficient of similarity-inclusion measures showed that the species composition of the swamps, artificial and natural ponds is included in the species composition of rivers; diatoms of natural and artificial form a similarity group with swamps.