We biopsied the iliac-crest cartilage and the upper fibular epiphyseal plates of seven achondroplastic children and the iliac crest of two more. Histologically the iliac-crest cartilage and its growth plate were nearly normal. The fibular growth plate had cartilage-cell clusters separated by wide septa of fibrous matrix which appeared to be very slowly and irregularly resorbed at the vascular front. Bone formation was normal in the iliac wing and was very stunted in the fibular metaphysis. Bone formed normally in the fibular head, at the insertions of the biceps tendon and collateral knee ligament, and in the periosteum. The peculiar skeletal development in the child with achondroplasia can be explained on the basis of these histological observations.