Published computed tomographic (CT) data from the holotype of the early tribosphenidan Slaughteria eruptens were reanalyzed to test a new hypothesis of dental formula in this taxon. Results indicate that Slaughteria had a premolar count of five, and that erupted (or erupting) teeth of the holotype all represent premolar loci—the p2, p3, dp4, and dp5 (a partially calcified p5 is identified below the dp5). This dental formula is supported by serial homology of cheek teeth between eutherians and metatherians and the pre-tribosphenic mammal Peramus. The presence of five premolars in Slaughters provides additional evidence that this tooth count is primitive for tribosphenidans as a whole. Slaughteria also presents an undocumented mode of tooth replacement, with a mixed alternating and sequential pattern. This pattern may represent the condition in other taxa with five premolars, such as Peramus and some early eutherians, with potential bearing on the polarity of tooth replacement at the base of the Tribosphenida.