Among social insects, each caste undertakes specific activities in a colony, and their morphologies are likewise specialized for each function. \textit{Inquilinitermes microcerus} (Silvestri 1901) is a termite species that occurs in South America as an obligatory inquiline in the nests of \textit{Constrictotermes cyphergaster} (Termitidae, Nasutitermitinae). The present study investigated the caste differentiation system of \textit{I. microcerus} in natural colonies from the semiarid Caatinga of northeastern Brazil. Eleven measurements were taken of different body parts of the larvae, workers, presoldiers, soldiers, nymphs, and alates for morphometric analyses; principal component analyses (PCA) were performed based on those parameters. The nymphal line comprised five instars, followed by alates, in agreement with the developmental pattern described for Termitidae species. The apterous line included two larval instars, followed by workers, presoldiers, and soldiers. Both workers and soldiers had only one instar each and consisted of both male and female individuals, showing no sexual dimorphisms. The general pattern of differentiation of \textit{I. microcerus} was identical to that of \textit{Inquilinitermes fur}. In relation to the patterns previously observed in other species of Termitinae, the greatest divergence was related to the numbers of worker instars, as the species of that subfamily generally show more than one worker instar.