Earlier studies (1972–1987) on the occurrence of unionid bivalves in the eutrophicated Lake Mikołajskie (Masurian Lakeland, NE. Poland) were continued in 1992–2008. Of the five recorded species (Unio tumidus, U. pictorum, Anodonta cygnea, A. anatina, and Pseudanodonta complanata), A. cygnea and P. complanata disappeared by 1987, whereas U. pictorum was observed sporadically at that time, and the unionid occurrence zone was markedly reduced. Beginning in 1992, a pronounced increase in abundance of U. tumidus was observed, and since 2002–2003, a decline of the other two species. Since 2003 the maximal original vertical distribution of U. tumidus (down to ca. 5 m) was re-established. The size and age structure of its population changed markedly during the study. No individuals of shell length exceeding 55 mm and aged more than 5–6 years were recorded since 1987. Between 1972 and 1987 one-year-old individuals prevailed, whereas in the following years the age structure changed slowly, and in 2008 two- to four-year- old bivalves were most frequent. The potential effects of changes in trophy and the effects of other factors, e.g. decline of Dreissena polymorpha, on changes in the occurrence of Unionidae in Lake Mikołajskie are discussed.