AbstractFemales of freshwater mussels spawn eggs which, after fertilization, are brooded until the developing larvae reach a stadium ready to infest a fish host in order to complete the obligate parasitic phase of their life. We investigated the duration of the brooding period in the endangered thick-shelled river mussel Unio crassus in two rivers in southern Poland and identified the stadia of glochidia development which could be used to assess current larval developmental stages. The eggs were fertilized in marsupia. The duration of the brooding period shortened significantly during the season, from 35 days in early spring to 9 days in summer. The break between consecutive broods was usually short but very variable: from almost no break to 17 days. 20% of brooding events inspected every other day were miscarried at different stadia of larval development. The recorded differences in brooding time may significantly influence the reproductive output of individuals and populations, depending on the thermal conditions, which may be affected by both anthropogenic impact and global climate change. The study offers a practical scheme for assessing female gravidity advancement in Unio crassus, which may be very informative for the future application of conservation measures in this endangered European species.