Studies of the locust twig borer,Ecdytolopha insiticiana(Zell.), were conducted in 2 geographic regions of Maryland which differed sizeably in elevation and length of growing season. The number of larval instars was established. Head capsule dimensions were statistically analyzed and grouped into 7 separable categories, representing 7 probable larval instars. Compared within instars, the larvae mined greater distances upward from the point of attack than downward. Distances of attacks from the tip of the tree stem were significantly greater with advanced larval instars. Larvae per burrow averaged 1.41 for the entire study and numbers per burrow were not significantly different for different instars.