Individuals with transtibial amputation can activate residual limb muscles to volitionally control robotic ankle prostheses for walking and postural control. Most continuous myoelectric ankle prostheses have used a tethered, pneumatic device. The Open Source Leg allows for myoelectric control on an untethered electromechanically actuated ankle. To evaluate continuous proportional myoelectric control on the Open Source Ankle, we recruited five individuals with transtibial amputation. Participants walked over ground with an experimental powered prosthesis and their prescribed passive prosthesis before and after multiple powered device practice sessions. Participants averaged five hours of total walking time, and received no visual feedback during practice. After the final testing session, participants indicated their prosthesis preference via questionnaire. Participants increased peak ankle power after practice (powered 1.02 ± 1.09 W/kg and passive 0.3 ± 0.13 W/kg). Additionally, participants generated greater ankle work with the powered prosthesis compared to their passive device (p=0.009, 148% increase). Although peak power generation was not different, participants preferred walking with a prosthesis under myoelectric control compared to their passive device. These results indicate individuals with transtibial amputation can walk with an untethered powered prosthesis under continuous myoelectric control and generate similar magnitudes in peak power to their passive prosthesis after minimal training.