Many gait pathologies (e.g. stroke, injury, joint replacement, amputation) result in both bilateral asymmetry and substantial metabolic and biomechanical costs. Despite this, the role of gait asymmetry during walking remains uncertain;​ gait asymmetry may either be caused by the pathology or may be adaptive, minimizing the costs associated with an already problematic situation. Here I asked, are there inherent costs to gait asymmetry independent of gait pathology and beyond that imposed by non-preferred step times? To answer this question, I measured the rate of metabolic energy expenditure and calculated mechanical power production while healthy adults walked symmetrically and asymmetrically at a range of step and stride times. I found that walking with asymmetric steps required more metabolic power than symmetric gait at corresponding step times. The positive mechanical power production increased with increasing asymmetry, paralleling the increases I observed in metabolic power. I suggest that the increased need for mechanical power may result from increased power absorption during double support and compensation through increased power production during single support. Overall, I identify an inherent metabolic and mechanical cost to gait asymmetry and find that symmetry is optimal in healthy, symmetric adults.