The identification of eight previously undescribed polypeptides in chicken embryo cells infected with Sindbis virus is reported. Seven of these polypeptides were distinguishable from the virus structural polypeptides and their precursors by their molecular weights and tryptic peptide maps. The eighth was closely related to pE2 (Schlesinger and Schlesinger, 1973), a precursor to one of the virus particle glycoproteins. Pulse-chase experiments and the use of an inhibitor of proteolytic cleavage allowed a division of the seven nonstructural (NS) polypeptides into three stable end products (NS p89, NS p82, and NS p60) and four precursors (p230, p215, p150, and p76). The labeling kinetics after synchronous initiation of translation indicated that synthesis of the NS polypeptides started at a single site and showed that the order of the genes coding for the NS polypeptides was (5' leads to 3') NS p60, NS p89, and NS p82. Short-pulse experiments under conditions of both synchronized and nonsynchronized translation suggested that cleavage of the primary translation product of the NS genes occurred only after its synthesis was completed and that the first cleavage removed the C-terminal polypeptide. From these and other experiments, we propose a detailed scheme for the synthesis and processing of Sindbis virus NS polypeptides.