Semigroups of operators in the commutant of the Volterra operator J: ƒ(x) → ∝₀^xf(t) dt on L₂(0, 1) are constructed and studied by means of a complex Fourier transform technique. A sufficient condition for a fixed operator in the algebra to be embeddable in a C₀ semigroup of operators in is given. The condition is shown to be necessary under the further restriction that the semigroup consists of contraction operators (see Theorem 3). In this case, a description of all C₀ semigroups of operators in containing the fixed operator is obtained (see Theorem 2). Under mild conditions, the domain of analyticity of a semigroup of operators in is shown to be a sector. Relationships between the lattices of closed invariant subspaces of the operators comprising a semigroup of operators in and the resolvents of the infinitesimal generator of the semigroup are studied.