  We check some consistency conditions for the D9-anti-D9 system in type I
string theory. The gravitational anomaly and gauge anomaly for SO(n) x SO(m)
gauge symmetry are shown to be cancelled when n-m=32. In addition, we find that
a string theory with USp(n) x USp(m) gauge symmetry also satisfies the anomaly
cancellation conditions. After tachyon condensation, the theory reduces to a
tachyon-free USp(32) string theory, though there is no spacetime supersymmetry.
