The study of stellar populations in external galaxies has so far concentrated on elliptical galaxies, spirals and a few dwarf galaxies, while S0 galaxies found only relatively little attention (with a few notable exceptions, e.g., Gregg, 1989; Bothun and Gregg, 1990). This is astonishing because S0-galaxies offer a unique possibility to study (a) old disk populations outside our own galaxy, (b) the properties of bulges in external galaxies without contamination by blue disk stars, and (c) the gradual change of the stellar populations of galaxies as a function of disk-to-bulge ratio. Last but not least, the comparison of the stellar populations of ellipticals, spirals and S0 galaxies allows to re-iterate on the question whether S0's are principally similar to spirals but have just stopped forming stars earlier, or, whether they form an class intermediate between ellipticals and spirals (e.g.Larson et al., 1980; van den Bergh, 1994).