  We have obtained resolved stellar photometry from Hubble Space Telescope
(HST) Advanced Camera for Surveys (ACS) observations of a field in the outer
disk of M81 as part of the ACS Nearby Galaxy Survey Treasury (ANGST). Motivated
by the recent discovery of extended UV (XUV) disks around many nearby spiral
galaxies, we use the observed stellar population to derive the recent star
formation histories of five ~0.5 kpc-sized regions within this field. These
regions were selected on the basis of their UV luminosity from GALEX and
include two HII regions, two regions which are UV-bright but Halpha-faint, and
one "control" region faint in both UV and Halpha. We estimate our effective SFR
detection limit at ~2 x 10^-4 Msun/yr, which is lower than that of GALEX for
regions of this size. As expected, the HII regions contain massive main
sequence stars (in the mass range 18-27 Msun, based on our best extinction
estimates), while similar massive main sequence stars are lacking in the
UV-bright/Halpha-faint regions. The observations are consistent with stellar
ages <10 Myr in the HII regions, and >16 Myr in the UV-bright/Halpha-faint
regions. All regions but the control have formed ~10^4 Msun of stars over the
past ~65 Myr. Thus, our results, for at least one small area in the outer disk
of M81, are consistent with an age difference being sufficient to explain the
observed discrepancy between star-forming regions detected in Halpha and those
detected exclusively in UV. However, our data cannot conclusively rule out
other explanations, such as a strongly truncated initial mass function (IMF).
