The wing and wing-associated muscles of the shoulder and thorax in the bird all cleave from common myogenic masses in the developing wing bud and are referred to collectively as brachial muscles. In this study the precise embryonic origin of the brachial muscles was determined using chick-quail chimaeras. Such chimaeras consisted of a graft of one somite taken from a 2-day quail donor embryo transplanted to the equivalent location in a 2-day chick host embryo. The chimaeras were analysed at 9.5-10.0 days in ovo to determine the location of the grafted cells and therefore the structures that were derived from the transplanted somite. The somites that were studied in this matter were somites 13 to 23 inclusive. The results show that only somites 16 to 21 inclusive contribute cells to the brachial musculature; moreover, the cells from a given somite are not distributed randomly among the brachial muscles but populate specific muscles only: thus it has been possible to map the somitic origin of individual brachial muscles. Moreover, there is an indication that each somite plays a unique role in the development of the brachial muscles.