Let $P$ be a pseudodifferential operator with symbol $p(x,\xi)$ belonging to $S^m_{1,0}$ and let $a(x)$ be a $C^\infty$ function. Then the operator $Pa$ defined by $Pa u=P(au)$ is a pseudodifferential operator of order $m$ with symbol $q(x,\xi)$ such that $$q-\sum_{\vert \alpha\vert\le N} \frac{1}{i^{\vert \alpha\vert}\alpha !}(\partial_\xi^\alpha p)(x,\xi) (\partial_x^\alpha a)(x)\in S^{m-N-1}_{1,0},$$ which provides an asymptotic expansion of the symbol of the operator Pa, following simply from the composition formula.