Histological examination of thymi from juvenile Acipenser ruthenus and Acipenser brevirostrum after haematoxylin and eosin staining revealed the presence of Hassall's corpuscles, in a scattered distribution within the thymic medulla in two of seven, and one of nine examined specimens, respectively. Morphologically, the Hassall's corpuscles were of the juvenile and premature type and no degenerative changes were seen.