In this paper a hidden Markov Model (HMM) technique for the estimation of the shape of a towed array is presented. It is assumed that there is a farfield source radiating sound containing possibly weak spectral lines. The technique uses either the Fourier coefficients at a given frequency computed from a single time block or the maximal eigenvector of a sample spectral covariance matrix. The technique is illustrated using several simulations. The results of these simulations indicate the HMM technique yields shape and bearing estimates more accurate than those provided by a maximum likelihood array shape estimation technique.