Mixed mode I+III loading of a fracture front results in out-of-plane propagation into echelon stepping fractures. Because a planar fracture geometry is the exception rather than the rule, and because the introduction of even a minor component of mode II or III loading is known to promote out-of-plane propagation, an understanding of mixed mode fracture growth is imperative to analyze fracture behavior. We have loaded cracks in mixed mode I+III within polymethyl methacrylate (PMMA or Plexiglas) rectangular blocks resembling conceptual fracture mechanics models of mixed mode loading and have analyzed the resulting geometries. The observed angle of twist of echelon fractures from the parent crack plane increases with the ratio KIII/KI and falls below theoretical predictions. Fracture propagation paths depend not only on the load ratio applied but also on sample geometry, loading configuration, and interaction among growing fractures. Sample geometry and loading configuration are approximately accounted for using analytical determinations of the stress intensity factors. We propose that interaction among growing fractures may contribute to the discrepancy between theoretically predicted twist angles and those observed in these and other mixed mode I+III experiments. Analysis of these experimental results has motivated the design of a new sample and loading configuration to test the propagation paths of uniformly loaded mixed mode I+III fractures. ¿ American Geophysical Union 1996