It has been shown that one of the 12 serine residues within the 23 kDa segment of myosin subfragment 1 can be covalently modified with a fluorescent probe 9-anthroylnitrile (ANN) [Hiratsuka, T. (1989) J. Biol. Chem. 264 (30), 18188-18194]. To identify the exact binding site of the probe, the distances between the bound ANN as donor and acceptors in known positions (Lys-553 or Cys-707) of the myosin head were determined by using fluorescence resonance energy transfer. Comparison of the spectroscopic results with distances obtained from the atomic model of subfragment 1 revealed that ANN binds to Ser-181. The result was in good agreement with the assumptions of Andreev and co-workers [Andreev, O. A., et al. (1995) J. Muscle Res. Cell Motil. 16 (4), 353-367]. This conclusion was further supported by protein modeling calculations. The results presented herein might bring ANN into the focus when the molecular mechanism and effects of the binding of ATP and its subsequent hydrolysis are studied.