1. Experienced touch typists were asked to type words in which only a single letter was typed by one hand, while the remaining letters were typed with the other hand. 2. Translational and rotational motion of each finger was computed optoelectronically from the location of reflective markers. Translational and rotational motion of both wrists was also computed from the locations of these markers. 3. Typically, when a subject typed a single letter, all of the fingers of the hand were in motion, as was the wrist. For each letter, this overall kinematic pattern of finger and wrist motion was highly repeatable. Thus the keystroke kinematics formed a repeatable signature for a particular letter typed by a particular subject. 4. During the keystroke the other hand was also in motion, typing the preceding and succeeding letters. During this period the motion of the two wrists and the motions of corresponding fingers of both hands was uncorrelated. 5. Because the keystroke kinematics are highly repeatable and independent of the movement of the contralateral hand, each keystroke represents a fundamental element of the typing movement. Thus these results provide a basis for determining the processes whereby sequences of keystrokes are assembled to type words.