  We show that two-dimensional SO(N) and Sp(N) Yang-Mills theories without
fermions can be interpreted as closed string theories. The terms in the 1/N
expansion of the partition function on an orientable or nonorientable manifold
M can be associated with maps from a string worldsheet onto M. These maps are
unbranched and branched covers of M with an arbitrary number of infinitesimal
worldsheet cross-caps mapped to points in M. These string theories differ from
SU(N) Yang-Mills string theory in that they involve odd powers of 1/N and
require both orientable and nonorientable worldsheets.
