Incremental lines of von Ebner are microstructural features that demarcate the daily apposition of dentin, such that the total number of incremental lines in a tooth serves as a measure of that tooth’s age. Differences in incremental line counts between successive replacement teeth within an alveolus provide a quantitative estimate of tooth replacement rate. Tooth formation times and rates have been shown to vary substantially among amniotes. We examined tooth replacement rates in two contemporaneous sauropod dinosaurs from western North America: Diplodocus and Camarasaurus. CT data reveal that each alveolus in Diplodocus has up to 5 replacement teeth, whereas Camarasaurushas a maximum of 3. Incremental lines of von Ebner are on average ~14 microns apart in Diplodocus and ~23 microns apart in Camarasaurus. Tooth formation times are about twice as long in Camarasaurus as in Diplodocus. Average tooth replacement rate in Diplodocus is ~35 days, similar to the replacement time of ~30 days found in Nigersaurus, which possessed a tooth battery. Tooth replacement rates in diplodocoid sauropods appear to be faster than those in any other dinosaurs, including derived ornithischian herbivores with tooth batteries. Preliminary data for Camarasaurusindicate a much slower rate of replacement. In the absence of incremental line counts from thin-sections in other sauropods, successive replacement tooth size was used as a proxy for replacement rate (i.e., a larger size discrepancy between successive replacement teeth suggests a slower replacement rate). Basal sauropods have fewer (1–2) replacement teeth per alveolus, and successive replacement teeth show a larger size difference than in Camarasaurus. In derived titanosaurs, which had narrow-crowned teeth, there are small size discrepancies between successive replacement teeth and a relatively large number of teeth per alveolus. Our data suggest that neosauropods are characterized by faster tooth replacement rates than basal sauropods, and that derived titanosaurs and diplodocoids independently evolved very fast tooth replacement rates