X-ray (CuK_α) scattering curves of rat liver mitochondria are characterized by continuously decreasing intensity from 0.5 to 5 mrad and a broad maximum centered near 20 mrad. The condensed-to-orthodox morphological transition of the inner membranes of intact mitochondria causes a dramatic decrease in scattering at very small angle and a marked shift of the 20 mrad maximum to smaller angle. A similar small-angle scattering maximum is observed with inner mitochondrial membrane fractions prepared by digitonin treatment and osmotic shock/step gradient centrifugation procedures. However, the small-angle X-ray scattering curves of mitochondria after acetone treatment and osmotic lysis/sonication are essentially continuous. These characteristics of mitochondrial X-ray scattering are discussed in terms of known structural features of the organelle.