In this paper we study the reflexivity of a unital strongly closed algebra of operators
with complemented invariant subspace lattice on a Banach space. We prove that if
such an algebra contains a complete Boolean algebra of projections of finite uniform
multiplicity and with the direct sum property, then it is reflexive, i.e., it contains
every operator that leaves invariant every closed subspace in the invariant
subspace lattice of the algebra. In particular, such algebras coincide with their
bicommutant.