The paintings can be divided into two different cycles belonging to different periods: a first cycle (in the apse, Southern wall, arch and inner side of the facade, was the work of the Cassinese-byzanthine school ; a second cycle was by Tuscan artists and covers the whole Northern wall. Then in the early 16th century the frescoes on the edicola were added.
The narrative begins on the wall right of the Triumph arch, and proceeds along the southern wall with episodes from the Genesis. The first episodes are dedicated to the creation and start from the 4th day, with a young beardless God separating the sun from the moon, then follows the creation of animals and birds.
Finally the creation of man, woman and Eden and a warning in Latin which reads (picture on the top, left):
Ex oi ligno Paradisi comede - de ligno Q E in medio Paradisi ne comedas - in Q hoc C M
= Ex oi (omni) ligno Paradisi comede - de ligno q (quod) e (est) in medio Paradisi ne comedas - in q (quocumque die) hoc (ex eo) c (comederis) m (morieris).
Translation: "Eat from any tree in Paradise - from the tree in the middle do not eat - on any day from it you should eat you shall die".
In the central section there are six Prophets with the divine message in their left hands. Other paintings in this section were lost when in 1597 a stone altar with the coat-of-arms of the town of Fossa was erected.
In the section on the left the paintings are in three rows, and include St. George killing the dragon and St. Martin giving his cloak to a poor beggar (photo on the top, right); both Saints are shown on white horses with garments contemporary to those of the knight who ordered the work to be done, probably a certain Guilelmus Morellus, represented in the Presyterium. Just below there is a magnificent calendar with the last 6 months of the year, each represented by a different character performing a farming activity typical of the month: in July wheat harvesting, in August threshing, in September fruit harvest, in October pressing grapes to make wine, in November sowing and in December killing the pig. Finally three giant Patriarchs (Abraham, Isaac and Jacob) holding in their arms smaller female figures with medieval hair styles and monks.