<p>The most important temple of Isis in Rome (and eventually, the Mediterranean) stood in the central *Campus Martius, immediately E of the *Porticus Meleagri, where it shared a sanctuary with an equally significant but separate Temple of Serapis. The construction of this great Iseum is often, though without good reason, attributed to Caligula. It is more likely, however, that the temple vowed by the second triumvirate in 43 B.C. (Dio Cass. 47.15.4: νεὼν τῷ τε Σαράπιδι καὶ τῇ Ἴσιδι) was in fact built, and that this was the Iseum Campense (Castagnoli).</p> <p>This temple was completely destroyed by fire in A.D. 80 and subsequently rebuilt by Domitian (Eutr. 7.23.5; Dio Cass. 66.24.2). The Severan Marble Plan fragments that show the large D-shaped courtyard of the Serapeum (Rodríguez Almeida, <i>Forma</i> frags. 35 m,s,t,u), are therefore of limited usefulness for reconstructing the architecture of its Augustan phase, as are the remains of its two monumental lateral entrance-ways. Assuming that its dimensions were not enlarged by subsequent rebuildings, we are able to assess the overall size of the original Iseum. A high concentration of Egyptian cult artifacts can be traced back to a rather large rectangular zone (<i>c</i>. 200 x 50 m) bounded by the transept of the Church of S. Maria sopra Minerva, Via del Seminario, Via di S. Ignazio, and Via Pié di Marmo (Roullet). Traces of the sanctuary’s N wall have recently been discovered under the Palazzo del Seminario (Alfano).</p>