Cryo-electron tomography was used to study the structural organization of whole frozen-hydrated mitochondria from Neurospora crassa. Unlike mitochondria from many other species and tissues, in this case the cristae form a three-dimensional network of interconnected lamellae. Basically, the three-dimensional structure of ice-embedded mitochondria from this species is consistent with previous descriptions of mitochondria prepared by chemical fixation and resin embedding. Nonetheless, ice-embedded mitochondria display some important differences: the outer surface of the mitochondria was found to be rather smooth, the intermembrane space was constant in width, and distinct contact sites between the membranes were clearly revealed. Furthermore ATP synthase particles on the outer surface of an "inside-out vesicle" were visible in 3-D reconstructions. Thus, cryo-electron tomography can provide detailed insights into these organelles with minimal perturbations of the physiological state. This indicates that it is a realistic goal to achieve "molecular resolution" with rather large biological specimens in the near future, ultimately allowing the identification and localization of macromolecules in their cellular context.