Lon (or La) is a soluble, homooligomeric ATP-dependent protease. Mass determination and cryoelectron microscopy of pure mitochondrial Lon from Saccharomyces cerevisiae identify Lon as a flexible ring-shaped heptamer. In the presence of ATP or 5'-adenylylimidodiphosphate, most of the rings are symmetric and resemble other ATP-driven machines that mediate folding and degradation of proteins. In the absence of nucleotides, most of the rings are distorted, with two adjacent subunits forming leg-like protrusions. These results suggest that asymmetric conformational changes serve to power processive unfolding and translocation of substrates to the active site of the Lon protease.