This article presents an interpretation of results of a geophysical survey campaign conducted in Ostia Antica, the principal port-town of the Roman Empire, in May 2015, which focused on the unexcavated area in the southwest part of the city. The surveys formed part of the ‘Neighbourhoods of Roman Ostia’ study conducted by Hanna Stöger (Stöger 2011; 2014). The project was committed to non-invasive methods in archaeology and aimed to reconstruct urban neighbourhoods from a long-term perspective. The investigated area runs parallel to the southern extent of Ostia’s cardo maximus, and lies directly behind the city blocks, IV, i-iv, and offers an important insight into the previously unknown extent of the city’s urban fabric. An additional targeted study was carried out in the Terme del Faro (IV, ii, 1).