Cellular progenitors of the coronary vasculature are believed to enter the chicken heart during epicardial morphogenesis between stages 17 and 27 (days 3-5) of egg incubation. To trace cells which give rise to the coronary arteries in vivo, we applied retroviral cell tagging procedures and analyzed clonal populations of vascular smooth muscle, endothelium, and connective tissue in the hearts of post-hatch chickens. Our data provide direct proof that (i) vascular smooth muscle progenitors begin to enter the heart at stage 17, substantially after the heart begins propulsive contractions; (ii) cardiac myocytes, vascular smooth muscle, perivascular fibroblasts, and coronary endothelial cells all derive from independent precursors when these cells migrate into the heart; (iii) endothelial cells of the coronary vessels have a different clonal origin than endothelial cells of the endocardium; (iv) coronary arteries form by the coalescence of discontinuous colonies (i.e., in situ vasculogenesis), each derived from a founder cell tagged at the time of retroviral injection (stages 17-18); and (v) coronary arteries contain discrete segments composed of "polyclones." These studies indicate the feasibility of gene targeting to coronary progenitors through the use of recombinant retroviruses.