Respiration and hydration of excised gill tissue were studied in Eriocheir sinensis adapted to fresh water and seawater, and in eyestalkless (ESR) crabs adapted to fresh water. Anterior red gills are morphologically and functionally different from posterior white gills. Red gills in fresh water have a greater fluid content than white gills. Red and white gills in seawater are similar. Freshwater-adapted white gills have the highest respiratory rate; seawater-adapted red gills have the lowest rate. Wet weight/dry weight comparisons between eyestalkless and normal crabs, of either red or white gills, show some differences. Fluid content of normal white gills is higher, and of normal red gills is Lower, when compared with comparable gills from ESR crabs. Respiration rates of both types of gills from eyestalkless crabs are higher when compared with gills from control crabs. Electron microscope studies support conclusions based on the respiration and hydration measurements. Cells of red gills in fresh water have characteristics of inactive cells, whereas those cells of white gills are metabolically active and are concluded to be responsible for salt regulation. The roles of the two gills are discussed relative to volume regulation, salt balance, changes in gill permeability, and neurosecretory control.