For trace class operators A, B ∈ B 1 (H) (H a complex, separable Hilbert space), the product formula for Fredholm determinants holds in the familiar form det H ((I H − A)(I H − B)) = det H (I H − A)det H (I H − B). When trace class operators are replaced by Hilbert-Schmidt operators A, B ∈ B 2 (H) and the Fredholm determinant det H (I H − A), A ∈ B 1 (H), by the 2nd regularized Fredholm determinant det H,2 (I H − A) = det H ((I H − A) exp(A)), A ∈ B 2 (H), the product formula must be replaced by det H,2 ((I H − A)(I H − B)) = det H,2 (I H − A)det H,2 (I H − B) × exp(− tr H (AB)). The product formula for the case of higher regularized Fredholm determinants det H,k (I H −A), A ∈ B k (H), k ∈ N, k 2, does not seem to be easily accessible and hence this note aims at filling this gap in the literature.