A 7-fold symmetric particle has been identified in Neurospora crassa which is most probably the mitochondrial chaperonin. The particle, about 12 nm in diameter, appears in preparations of cytochrome reductase, and is shown to contain a 60 kd protein which cross-reacts with anti-GroEL antibodies. Results of STEM mass measurement suggest that the particle is composed of 14 subunits. A preliminary interpretation of the structure of the particle based on electron microscopy is given. Its quaternary structure and molecular weight are similar to those of the recently discovered family of particles called chaperonins, found in bacteria, chloroplasts and mitochondria.