We have devised a procedure for evaluating the absolute magnitudes of galaxies from their optical rotation curves, as an extention of the conventional Fisher-Tully method. We describe here how this method can be employed to evaluate the Hubble constant. From observations of 23 Sb field galaxies with luminosities ranging from –19.5 to –23.0 (adopting H=50 km s–1 Mpc–1), we have produced synthetic rotation curves showing the systematic progression toward increasing velocity with increasing luminosity within a given Hubble type. (See Thonnard and Rubin, Carnegie Yrbk 80, p. 551 for details of producing such a set of curves). By matching even a small portion of a rotation curve with these curves, the absolute magnitude of an Sb galaxy can be estimated to about ±0.5 mag. This magnitude, of course, is based on an assumed value for H.