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The study investigated seasonal dynamics of the ostracod population in the Oder River (Poland), in the area of groyne fields. Groyne fields were lenitic areas in that river system, providing favourable conditions for ostracod development. The main aims of the study were to find information on seasonal differences in ostracod occurrence, determine models of density dynamics and become acquainted with the effect of environmental factors on seasonal changes of ostracod assemblages. Samples were collected from 15 research stations in the littoral. The respective zones of the groyne fields abounded in various microhabitats. Average density, frequency coefficient, dominance, Shannon-Wiener diversity index (H), and Pielou’s evenness index (J) were calculated for Ostracoda assemblages in order to point out differences between seasons. In total, 237 samples were collected; forty-seven species were identified, and the average density was 658 indiv. was studied during the vegetation season. The highest average density of the ostracods was discovered in spring (1160 indiv. m-2) with maximum of 9472 indiv. m-2.); in the subsequent seasons it dropped by about half. The density dynamics of particular Ostracoda species was often different from their overall density. Significant differences between seasons were discovered with regard to the number of taxa, density and evenness index. The species that dominated throughout the year included Limnocythere inopinata and Physocypria kraepelini. Furthermore, juvenile Candoninae dominated in spring, Cypridopsis vidua dominated in summer, and Candona neglecta, C. vidua and juvenile Pseudocandona dominated in autumn. It turned out that the dynamics of occurrence of certain species observed in the present study contradicted data from literature related to the following species: Darwinula stevensoni, Cypria ophtalmica, Physocypria kraepelini, Limnocythere inopinata. All obtained models of density dynamics pointed out maximum density in spring, indicating that during that period the conditions in the river were optimal for ostracod development, as confirmed by the results of studying environmental factors. Ecological condition of the Oder was also good in the other seasons. Temperature, visibility, and dissolved oxygen content were among factors which most strongly affected ostracod assemblages. Availability of organic matter could result in decreasing their density.