( 2001. The American Astronomical Society. All rights reserved. Printed in U.S.A. STELLAR MASS-TO-LIGHT RATIOS AND THE TULLY-FISHER RELATION

We have used a suite of simpliÐed spectrophotometric spiral galaxy evolution models to argue that there are substantial variations in stellar mass-to-light (M/L) ratios within and among galaxies, amount-ing to factors of between 3 and 7 in the optical and factors of 2 in the near-infrared. Our models show a strong correlation between stellar M/L and the optical colors of the integrated stellar populations. Under the assumption of a universal spiral galaxy initial mass function (IMF), relative trends in model stellar M/L with color are robust to uncertainties in stellar population and galaxy evolution modeling, including the eects of modest bursts of star formation. Errors in the dust-reddening estimates do not strongly aect the Ðnal derived stellar masses of a stellar population. We examine the observed maximum disk stellar M/L ratios of a sample of spiral galaxies with accurate rotation curves and optical and near-infrared luminosity proÐles. From these observed maximum disk M/L ratios we conclude that a Salpeter IMF has too many low-mass stars per unit luminosity but that an IMF similar to the Salpe-ter IMF at the high-mass end with less low-mass stars (giving stellar M/L ratios 30 % lower than the Salpeter value) is consistent with the maximum disk constraints. Trends in observed maximum disk stellar M/L ratios with color provide a good match to the predicted model relation, suggesting that th