  Let $Tr$ denote the trace $\mathbb{Z}$-module homomorphism defined on the
ring $\mathcal{O}_{L} $ of the integers of a number field $L.$ We show that
$Tr(\mathcal{O}_{L})\varsubsetneq \mathbb{Z}$ if and only if there is a prime
factor $p$ of the degree of $L$ such that if $\wp _{1}^{e_{1}}...\wp
_{s}^{e_{s}}$ is the prime factorization of the ideal $p\mathcal{O}_{L}$ in
$\mathcal{O}_{L},$ then $p$ divides all powers $e_{1},...,e_{s}.$ Also, we
prove that the equality $Tr(\mathcal{O}_{L})=\mathbb{Z}$ holds when $L$ is the
compositum of certain number fields.
