The present study investigated the caste differentiation system of Armitermes holmgreni in natural colonies from the Atlantic Forest in northeastern Brazil. Thirteen measurements were made of the termites encountered in three colonies. The morphometric differences among the instars were analyzed using principal component analysis (PCA). The differentiation patterns of the nymphal lineages of A. holmgreni agreed with earlier descriptions of species of the Termitidae family, with one larval instar followed by five nymphal instars and alates. Nymphoid neotenics became differentiated starting at the fourth or fifth nymphal instar. Two larval instars were observed in the apterous lineage before the appearance of workers, presoldiers, and soldiers. Both workers and soldiers had only one instar each. The A. holmgreni workers consisted of male and female individuals, without sexual dimorphism, a situation that had not been previously recognized among the Syntermitinae. The A. holmgreni soldiers consisted only of female individuals, a pattern observed in most species of Termitinae and Macrotermitinae. The differentiation of nymphoid neotenics starting at the fourth or fifth nymphal instar has also been reported for Armitermes euamignathus. The general patterns of differentiation of A. holmgreni were similar to those previously observed in other species of Syntermitinae.