The rates of the translational motion of myosin fragments, heavy meromyosin (HMM), and heavy meromyosin subfragment‐1 (HMM S‐1) were measured during actin‐activated ATPase reaction by the method of fluorescence correlation spectroscopy. This technique monitors the random fluctuations in the concentration of fluorescent molecules in an open volume which result from the translational diffusion of the molecular species under observation. The statistical behavior of the fluctuations is represented in the form of the autocorrelation function, which is related to the translational diffusion coefficient of the fluorescent molecules. The translational motion of fluorescently labeled myosin fragments was progressively slowed down after additions of increasing amounts of actin in the presence of excess MgATP. When these results are interpreted according to a simple binding scheme, the extent of the retardation can be used to obtain the apparent association constant for binding of S‐1 and HMM to actin in the presence of MgATP. In 0.1M KCl and at 23°C, the apparent association constants were determined as KappHMM = 2.2 × 104M−1 and KappS‐1 = 8.8 × 103 for HMM and S‐1, respectively.