Transmission electron microscopy of myotomal muscle from goldfish (Carassius auratus L.) reveals significant differences in distribution and density of mitochondria in animals acclimated to cold temperature (5°C) as compared to those acclimated to warm temperature (25°C). Micrographs of red and white muscle from these animals have been analyzed quantitatively for the stereological parameters Vv (volume density), Sv (surface density), and S N (specific surface); and diffusion distances between mitochondria have been calculated as the harmonic mean of spacings between clusters of mitochondria (711). The most dramatic differences occur in vv, sv, and 7h of red muscle, v v and Sv being significantly greater in cold-acclimated animals and 7h being smaller. In white muscle, Vv is significantly greater for cold-acclimated animals than warm-acclimated animals, but other parameters are not significantly different. Using our values of Sv as estimates of the surface area through which exchange of metabolites occurs and our values of Th as estimates of the distance over which diffusion occurs, we calculate that the diffusivity constant in the one-dimensional diffusion equation for metabolites may be reduced in cold-acclimated animals to as low as 0.29 that of warm-acclimated animals while still maintaining absolute rates of diffusive flux between cytoplasm and mitochondria. It appears reasonable to assume that cold-induced rises in the concentration of enzymes in fish muscle reflect changes in organellar densities-changes which may serve primarily to ameliorate the effect of temperature on diffusive exchange between mitochondria and cytoplasm rather than solely to overcome catalytic limitation as is widely assumed.