We consider the Fréchet * -algebra L(s , s) ⊆ L( 2 ) of the so-called smooth operators, i.e. continuous linear operators from the dual s of the space s of rapidly decreasing sequences to s. This algebra is a non-commutative analogue of the algebra s. We characterize closed * -subalgebras of L(s , s) which are at the same time isomorphic to closed * -subalgebras of s and we provide an example of a closed commutative * -subalgebra of L(s , s) which cannot be embedded into s.