  We present an I-band Tully-Fisher relation (TFR) for 18 nearby S0 galaxies
using kinematics derived from long slit spectroscopy of stellar absorption
lines. Our estimates of the circular velocity, V_c, at 2-3 exponential disk
scale lengths account for line-of-sight projection and for the stellar random
motions through an asymmetric drift correction. Uniform and accurate distance
calibration for all objects is available from surface brightness fluctuation
measurements of Tonry et al. (1998). Despite the care taken in estimating both
V_c and M_I, the TFR shows an intrinsic scatter, $\sim 0.7$ mag in M_I, or 0.15
in log_10(V_c). This result is surprising, as S0 galaxies appear to have both
the simple kinematics of disk galaxies, and the simple stellar populations of
early-type galaxies. Remarkably, in this sample of overall rotation-dominated
galaxies, the central stellar velocity dispersion is a better predictor of the
total I-band luminosity (through the Fundamental Plane relations) than the
circular speed at several exponential scale lengths. Furthermore, the TFR
zeropoint, or the mean stellar I-band luminosity at a given V_c, differs by
only $\sim 0.5$ mag between our sample of S0s and Mathewson et al.'s (1992)
sample of late-type spirals, once both data sets are brought onto a consistent
distance scale. This offset is less than expected if S0s are former spiral
galaxies with prematurely truncated star-formation ($\gtorder 4$ Gyrs ago).
