The Shops of the Fishmongers were installed on the Decumanus Maximus, in the north porticus of the Macellum (IV,V,2), the central market for fish, meat and vegetables. They are an addition from the first half of the third century AD (opus vittatum). They are situated on one of Ostia's busiest intersections, the Crossroads of the Castrum. Across the street are more fish shops, in the House of the Harbour Mosaic (I,XIV,2). The building was excavated in the late 1920's and in 1938.

There are two shops, to the west and east of the entrance corridor of the Macellum. In the centre of both rooms is a marble table, and against the back wall a fish-basin revetted with marble, supporting small columns. In the eastern shop is a black-and-white mosaic, with marine motifs. We see a triton and a dolphin with an octopus in its beak. Above and below the dolphin is the text:

It is an apotropaic text that, according to Giovanni Becatti, refers to the dolphin, who scares away the fish. Katherine Dunbabin suggests that it is rather a reference to the octopus, an animal that is traditionally hostile to man, contrary to the dolphin. Her view seems to be supported by a mosaic in other fish shops, the House of the Jealous One (V,V,1). Here we read INBIDIOSUS, but a dolphin has not been preserved.