Let Ω be a domain in the complex plane bounded by m+1 disjoint, analytic simple closed curves and let $z_0,...,z_n$ be n+1 distinct points in Ω. We show that for each (n+1)-tuple $(w_0,...,w_n)$ of complex numbers, there is a unique analytic function B such that: (a) B is continuous on the closure of Ω and has constant modulus on each component of the boundary of Ω; (b) B has n or fewer zeros in Ω; and (c) $B\left(z_j\right)=w_j$, 0 ≤ j ≤ n.