Four patrician flamines maiores appear in the center of the S frieze of the Ara Pacis, in a group between the figures of Augustus and Agrippa (fig. 1). These are thought to represent the flamines Dialis, Martialis, Quirinalis, and Iulialis, who were in charge, respectively, of the worship of Jupiter, Mars, Quirinus (the deified Romulus), and (after 42 B.C.) of the deified Julius Caesar. Following the death of Augustus in A.D. 14, a fifth (Augustalis) was added for his cult. The presence of the four Augustan flamines on the S frieze has been considered a historical crux. The current consensus is that the procession represents a general religious celebration of thanksgiving (supplicatio) in 13 B.C., on the occasion of Augustus's return from an extended stay in Spain and Gaul, rather than another one in 9 when the altar was dedicated. But Tacitus and Dio report that the office of flamen Dialis was vacant between the suicide of Cornelius Merula in 87 and the appointment of Servius Cornelius Lentulus Maluginensis in 11 B.C. Therefore, the presence of the fourth priest seems to argue against the frieze's portrayal of an event in 13. In order to resolve this problem, G. Bowersock argued on textual grounds that Servius Maluginensis became flamen Dialis in 14, not 11 — an idea that has not won wide acceptance. I suggest instead that there is a technical, sculptural explanation for the inclusion of the fourth flamen.