Thiobios (sulfide-dependent) meiofauna have an aerobic metabolism, are sulfide-insensitive and rarely contain symbiotic sulfur-oxidizing bacteria. Adaptations so far observed in thiobios are consistent with the overriding need to maximize the diffusive flux of oxygen and sulfide to optimize energy yield. Both accrue from the requirement that oxygen and sulfide uptake and metabolism be controlled by diffusion and first-order rate reactions. These adaptations include variation in shape, mitochondrial distribution and respiratory capacity. Meiofauna can substantially supplement a more inefficient anaerobic metabolism with aerobic metabolism at oxygen concentrations as low as 0.1 μM. Hence, an aerobic capacity is useful even in thiobios. Adaptation to low oxygen, by decreasing metabolic rate, is counterproductive unless oxygen consumption remains zero-order. Otherwise, aerobic metabolic capacity must be increased to maximize oxygen flux. Sulfide-insensitivity is necessary because a sulfide detoxification system running at physiologically reasonable rates cannot maintain internal sulfide concentration below toxic levels. Sulfide metabolism is probably energetically useful where it occurs. Few symbiotic relationships among meiofauna also reflect constraints imposed by diffusional gradients. Consideration of diffusive flux is necessary for interpreting physiological and biochemical data on oxygen and sulfide metabolism in diffusion-dominated animals such as most meiofauna.