Previous studies have investigated the use of head capsule widths from various insect species to describe the distributions of each instar. We present a unique method to classify instars of the gypsy moth,Lymantria dispar(L.), based on head capsule width. Our method uses nonlinear least squares parameter estimation to describe the distributions of each gypsy moth instar. Head capsule measurements from 3,903 field collected larvae were used to derive instar class ranges. Class boundaries were assigned at the point of distribution overlap between adjacent instars. Probabilities of misclassification were determined from probability distributions fitted to each instar. A final classification rule was developed that will provide managers with a tool for determining the phenological distribution of a gypsy moth population from field collected data. The distribution of field collected frass widths was also evaluated as a means for classifying instars and did not result in a useful instar classification method.