Objectives: The strengthening of the El Niño Southern Oscillation (ENSO) in the mid-Holocene caused significant changes in climate, vegetation, and faunal assemblages in South Australia. The appearance of a light, flexible backed-artifact toolkit ∼4 kya has been interpreted as evidence for changes in foraging behavior in response to this event. Optimal foraging theory supports a risk minimization strategy for South Australian hunter-gatherers in which increased mobility was used to cope with effects of a dryer, unstable environment in the late Holocene. Whether this event caused changes in foraging mobility will be tested by examining lower limb external diaphyseal shape between pre-ENSO and post-ENSO skeletons from Roonka Flat, South Australia. Materials and Methods: Anteroposterior and mediolateral diameters were used to construct diaphyseal shape indices for Roonka Flat femora and tibiae. If populations living in South Australia became more mobile over time, then post-ENSO skeletons should exhibit higher shape indices. Results: The pooled-sex post-ENSO sample has significantly higher femoral shape indices than the pre-ENSO sample. Males do not show significant diaphyseal shape differences over time, but females significantly increase. Discussion: These data are consistent with the risk minimization model, indicating that South Australians became more mobile post-ENSO to better exploit a less productive environment by expanding their foraging radii. The temporal shift toward more elliptical diaphyses is more notable in females than males, which is consistent with Aboriginal ethnographies that show both sexes being intensely involved in hunting and capturing game animals. Am J Phys Anthropol 161:94–103, 2016.