A size series of 143 specimens of the Permo-Carboniferous temnospondyl Archegosaurus decheni from the Saar-Nahe Basin (south-west Germany), ranging in size from 18 to 280 mm skull length, can be interpreted as an extensive growth series. This study focuses on the reconstruction of ossification sequences and developmental patterns of skull, visceral skeleton, and postcranium in A. decheni. With respect to the dermal skull, the ossification of the postcranium is delayed. The basibranchial starts to ossify after formation of the dermal pectoral girdle, sclerotic ring, stapes, scales, and scapulocoracoid. Initial ossification of the ischium takes place distinctly later than that of the ilium, and the pubis remains entirely cartilaginous. Vertebral centra ossify late, after formation of the ischium. Branchial dentition is resorbed at a skull length of 150 mm, indicating a long larval phase. Carpals and tarsals start to ossify only in the largest individuals. Archegosaurus decheni exhibits no period of obvious metamorphosis, as indicated by the dissociation of developmental events during ontogeny. The delayed formation of scales, scapulocoracoid, and basibranchial, in comparison with the Permo-Carboniferous temnospondyls Sclerocephalus and Onchiodon, may be correlated with the secondary aquatic adaptation of A. decheni throughout its life history. The chronology of ossification of the endocranial elements can be reconstructed. The exoccipital ossifies first, followed by the quadrate, basioccipital, basisphenoid, sphenethmoid, and otic capsule. Comparison with other anamniotic tetrapods shows that the early ossification of the exoccipital and quadrate is a stable pattern, and that there is a tendency to eliminate ossification of the basioccipital and basisphenoid.