The estimated lower limb length (0.761–0.793 m) of the partial skeleton of Australopithecus afarensis from Woranso-Mille (KSD-VP-1/1) is outside the previously known range for Australopithecus and within the range of modern humans. The lower limb length of KSD-VP-1/1 is particularly intriguing when juxtaposed against the lower limb length estimate of the other partial skeleton of A. afarensis, AL 288-1 (0.525 m). A sample of 36 children (age, >7 years, trochanteric height = 0.56–0.765 m) and 16 adults (trochanteric height = 0.77–1.00 m) walked at their self-selected slow, preferred, and fast walking velocities, while their oxygen consumption was monitored. Lower limb length and velocity were correlated with slow (P < 0.001, r2 = 0.44), preferred (P < 0.001, r2 = 0.55), and fast (P < 0.001, r2 = 0.69) walking velocity. The relationship between optimal velocity and lower limb length was also determined and lower limb length explained 47% of the variability in optimal velocity. The velocity profile for KSD-VP-1/1 (slow = 0.73–0.75 m/s, preferred = 1.08–1.11 m/s, and fast = 1.48–1.54 m/s) is 36–44% higher than that of AL 288-1 (slow = 0.53 m/s, preferred = 0.78 m/s, and fast = 1.07 m/s). The optimal velocity for AL 288-1 is 1.04 m/s, whereas that for KSD-VP-1/1 is 1.29–1.33 m/s. This degree of lower limb length dimorphism suggests that members of a group would have had to compromise their preferences to walk together or to split into subgroups to walk at their optimal velocity. Am J Phys Anthropol, 2012. © 2012 Wiley Periodicals, Inc.