Abstract. Let K be a number field and p a prime number ≥ 5. Let us denote by µ p the group of the pth roots of unity. We define p to be K-regular if p does not divide the class number of the field K(µ p ). Under the assumption that p is K-regular and inert in K, we establish the second case of Fermat's Last Theorem over K for the exponent p. We use in the proof classical arguments, as well as Faltings' theorem stating that a curve of genus at least two over K has a finite number of K-rational points. Moreover, if K is an imaginary quadratic field, other than Q( √ −3 , we deduce a statement which allows often in practice to prove Fermat's Last Theorem over K for the K-regular exponents.