  We present ultraviolet photometry (NUV band, 180--280 nm) of 405 asteroids
observed serendipitously by the Galaxy Evolution Explorer (GALEX) from
2003--2012. All asteroids in this sample were detected by GALEX at least twice.
Unambiguous visible-color-based taxonomic labels (C type versus S type) exist
for 315 of these asteroids; of these, thermal-infrared-based diameters are
available for 245. We derive NUV-V color using two independent models to
predict the visual magnitude V at each NUV-detection epoch. Both V models
produce NUV-V distributions in which the S types are redder than C types with
more than 8-sigma confidence. This confirms that the S types' redder spectral
slopes in the visible remain redder than the C types' into the NUV, this
redness being consistent with absorption by silica-containing rocks. The GALEX
asteroid data confirm earlier results from the International Ultraviolet
Explorer, which two decades ago produced the only other sizeable set of UV
asteroid photometry. The GALEX-derived NUV-V data also agree with previously
published Hubble Space Telescope (HST) UV observations of asteroids 21 Lutetia
and 1 Ceres. Both the HST and GALEX data indicate that NUV band is less useful
than u band for distinguishing subgroups within the greater population of
visible-color-defined C types (notably, M types and G types).
