In the Bichi Chapel (the first in the right transept) is a monochrome fresco Nativity painted in the second half of the fifteenth century by the Sienese artist Francesco di Giorgio Martini. He sets the scene in a shed among the rocks; inside it is Jesus, born to bring peace to mankind.
The Nativity is also the subject of another painting, a canvas by the Viterbo artist Giovanni Francesco Romanelli, made around 1640 and hanging above the fourth altar on the right in the nave.