Intercastes of the slave-making ant Polyergus rufescens (Hymenoptera, Formicidae) were morphometrically analyzed and compared with queens and workers castes. Clear cut differences among these three 'morphological' castes were recognized. Results of multivariate analysis showed significant differences in mean size for most morphological characters. On the basis of 15 external features we measured, intercastes were more similar to workers but distinguishable from them by a larger head and a queen-like abdomen (petiole and gaster). Canonical discriminant analysis also indicated this morphological differentiation. Mahalanobis' distances were 21.65 between queens and workers, 20.26 between queens and intercastes, and 6.02 between intercastes and workers.