  In this article, we establish the Hopf-Tsuji-Sullivan dichotomy for geodesic
flows on certain manifolds with no conjugate points: either the geodesic flow
is conservative and ergodic, or it is completely dissipative and non-ergodic.
We also show several equivalent conditions to the conservativity, such the
Poincar\'e series diverges at the critical exponent, the conical limit set has
full Patterson-Sullivan measure, etc.
