This aisle is the scene of the first great archeological discoveries of Fr Joseph Mullooly, O.P., in 1857. The hole in the wall through which he first entered this level is still visible.

This niche was probably part of a small altar or shrine, and was discovered with frescoes on the back, the sides and the vault. The back wall proved to be of particular interest and carried a fresco of the Madonna and Child. However, it was in a very precarious state, and after a few days fell off the wall completely―only to reveal the present Madonna and Child in an almost perfect state of preservation.

Of all the surviving frescoes in the fourth-century basilica, this is one of the oldest. The central figure of the Madonna and Child is flanked on the side walls by two saints, perhaps St Euphemia and St Catherine of Alexandria, both wearing crowns of pearls. The Madonna wears a more elaborate royal diadem of pearls and precious stones, of a typically Byzantine inspiration. Nonetheless, this fresco is a fine example of typically Western and specifically Roman iconography.

All along the wall there are some fragmentary frescoes, one of which appears to depict the Universal Judgement.