Let V denote a Volterra integration operator, acting in the space [image omitted] of analytic functions [image omitted] for which [image omitted] by [image omitted] Using the Duhamel product method, it is shown that for any positive integer k the sequence {(kn)!Vknf}n epsilon 0 is a basic sequence in [image omitted] equivalent to the basis [image omitted] if and only if f(0)0.