Abstract
The purpose of this work is the designation of changes in the environmental preferences of mayflies in the prevailing conditions in a large lowland river little changed by human activity. During the study, the occurrence of the following species were recorded: Cercobrachys minutus (54.2%); Procloeon nana (24.5%); Pseudocentroptiloides shadini (7.6%); Ametropus fragilis (2.9%), which were numerously represented and formed a permanent element of the fauna, and Baetopus wartensis (0.1%) and Oligoneurisca borysthenica which were noted very rarely and were accidental species. The differences and similarities in the functionality of the psammophilous and other mayfly assemblages were assessed. The data from the last fifteen years of study was investigated and compared with those from other rivers which were more managed. PCA was used to distinguish the main factors influencing habitat selection at particular stages of life history and the tendency for drifting.