The effects of thyroid hormone (TH) level changes on skeletal development were studied in cyprinids Labeobarbus intermedius (Barbinae) and Danio rerio (Danioninae). TH was revealed to influence development of a skull, axial skeleton, paired fins and girdles, and squamation. Usually, high TH-level promotes the premature onset and acceleration of ontogenetic processes, whereas TH-deficiency delays and retards them. However, skeletal elements differ in TH-responsiveness. Alterations of TH level induce developmental changes in some structures but do not cause any changes in others. As a result, the normal sequence of ontogenetic events and the temporal interrelations between ontogenetic processes undergo changes resulting in changes in adult morphology.