Approximately 9,100 species belonging to the staphylinid beetle subfamily Pselaphinae have been described based on adults, but the larvae of only 19 species and 15 genera have been described in sufficient detail for systematic study, and the pupa of only a single species has been described. Larvae of the euplectine tribe Mayetiini (152 species, all in the genus Mayetia) have not been described previously. The larva and pupa of Mayetia pearsei Schuster et al. are described herein based on two larvae, a cast exuvium, and five pupae from soil wash samples containing numerous adults of the species. Larvae are distinguished from other described larvae of pselaphines based on the following combination of characters: eversible frontal vesicle simple, sac-like, with fine, dense terminal papillae; stemmata absent; epicranial stem absent; antenna two segmented, segment two bearing unbranched, elongate, sinuate sensorium; mesothoracic spiracle with large, fusiform peritreme; abdominal spiracles absent; A9 with V-shaped, apical emargination between short, fixed urogomphi. Pupae are similar to those of the European pselaphine Plectophloeus fischeri (Aubé) in having elongate prothoracic setate processes and spinose tubercles covering the antennae. The pupal description may represent the smallest coleopteran pupa described to date. Larvae and pupae of M. pearsei were found during July and August, whereas adults were found July-October in loamy sand in the floodplain of the Meramec River, Crawford County, MO.