Major and minor chelae of Carcinus maenas were investigated with respect to mechanical advantage, angles of pennation and sarcomere lengths of the closer muscle fibres, maximum pinching forces induced by nervous stimulation and maximum stress in the closer muscles. Major chelae differed from minor chelae in all these factors. Mean mechanical advantages of major and minor chelae were 0·36 and 0·26, respectively. Angles of pennation were greater at the distal ends of closer muscles and declined proximally: in half open major and minor chelae distal to proximal angles were about 35–26° and 29–19°, respectively. Mean sarcomere lengths in closer muscles of fully closed major and minor chelae were significantly different at 7·6 and 6·0 fim, respectively; there was great variation within chelae and between crabs. Major chelae pinched more strongly than minor chelae due to higher mechanical advantages, larger closer apodemes and greater stress in the muscle. Mean maximum stresses in major and minor chelae were 677 and 474 kN/m2, respectively. The biggest stress recorded–1057 kN rrr2–was in a major chela. Pinching forces induced by injecting a high K+ and caffeine ringer were about 20% weaker than nervously stimulated pinches.