The writings of the 4th century bishop Ambrose of Milan are filled with borrowings from the 1st century Jewish philosopher Philo of Alexandria. While the nature of these usages within Ambrose’s five so-called Philonic treatises has been well studied, the existing scholarship on the use of Philo in the letters of Ambrose is scant. Within the collection of Ambrose’s letters, 16 of them make extensive use of a Philonic treatise. This study seeks to provide some general conclusions about the nature of Ambrose’s adaptational choices within this corpus. The way in which Ambrose handles his source reveals a thinker who holds Philo (and the tradition that he represents) in the highest esteem. Nonetheless, he is willing to dialogue with his source, alter it for his own purposes, and bring out the elements that are most suited to his own faith and context.