Elimination of limited areas of the cephalic neural crest in stage 9 or 10 chick embryos markedly reduced the size of the thymus gland or resulted in its absence. Small thymic lobes contained both thymocytes and epithelial cells but showed delayed development. Parathyroid and thyroid glands sometimes were reduced in size or missing from the normal location on one or both sides. Heart defects were consistently present. Thymus development may depend on direct interaction of mesenchymal derivatives of the neural crest with pharyngeal epithelium. Multiple defects, such as the Di George syndrome, may result from failure of neural crest derivatives to migrate and interact properly.