Pheidole obscurithorax is a member of the New World (and polyphyletic, see Moreau 2008) P. fallax species group defined by Wilson (2003). It is a large dark species over 6 mm in body length. The species is easily distinguished from P. megacephala by the much larger body size and relatively reduced postpetiole, in addition to the strongly sculptured head of the major worker (Fig. 12), and the much longer antennal scapes of the minor. It is separated from other New World species treated here, including those of the P. punctatissima clade and P. flavens complex, by the much larger size, prominence on the posterior slope of the promesonotum (Fig. 5, major; Fig. 50, minor), densely rugoreticulate face of the major (Fig. 12), and smooth head and long antennal scapes of the minor. The Old World species P. fervens, P. indica, and P. noda all have majors with strongly sculptured head and minors with smooth heads, and the reader is referred to the key for characters used to separate these from P. obscurithorax.