The present study suggests evolutionary changes in the morphology and size of the lower third premolar of the leporid Serengetilagus praecapensis from the Upper Ndolanya and Upper Laetolil Beds, Laetoli, Tanzania (ca. 3.85–2.66 Ma). Mandibular depth at p3 was compared also as a proxy indicator of size. The occlusal morphology of p3s from Laetoli is variable, but most commonly the tooth is crescentic with a posteroexternal reentrant (PER) extending about half way across the width of the tooth, plus distinct anteroexternal (AER) and anterior (AR) reentrants. An anterointernal reentrant (AIR) is weak to distinct. A proportionally higher percentage of p3s from the Upper Ndolanya Beds (50%) and the uppermost Upper Laetolil Beds (ULB, between Tuff 7 and the Yellow Marker Tuff, 49%) had a weak AIR compared to only 29% of specimens from between Tuffs 5–7, ULB. The higher frequency of a weak AIR in the geologically younger population is interpreted as the character state being newly reversed to the plesiomorphic condition (AIR weak to absent). There are only two poorly preserved p3s from the Lower Laetolil Beds: on both specimens the AIR and AR are weak to absent (plesiomorphic condition). AR is almost always present on p3s from the Upper Ndolanya and Upper Laetolil Beds. On average, p3s from the Upper Ndolanya Beds are slightly shorter and narrower, and the mandibles slightly less deep at the level of p3 than those from the Upper Laetolil Beds. However, the range of variation of measurements is quite similar between samples from the Upper Ndolanya and Upper Laetolil Beds. A specimen from the Upper Ndolanya Beds (EP 1223/03.1) has a p3 proportionally wider than mean values for other specimens from both the Upper Ndolanya and Upper Laetolil Beds. In conjunction with p3 occlusal morphology, this specimen may represent a new, as yet unnamed, species. Although interesting, the differences observed between samples from the Upper Ndolanya Beds and subunits of the Upper Laetolil Beds are not considered adequate for separation into a distinct species or subspecies.