What is needed of a tachyon if it is to be the dark energy?

We study a dark energy scenario in the presence of a tachyon field phi with potential V(phi) and a barotropic perfect fluid. The cosmological dynamics crucially depends on the asymptotic behavior of the quantity lambda=-MpVphi/V-3/2. If lambda is a constant, which corresponds to an inverse square potential V(phi)proportional tophi(-2), there exists one stable critical point that gives an acceleration of the Universe at late times. When lambda-->0 asymptotically, we can have a viable dark energy scenario in which the system approaches an instantaneous critical point that dynamically changes with lambda. If |lambda| approaches infinity asymptotically, the Universe does not exhibit an acceleration at late times. In this case, however, we find an interesting possibility that a transient acceleration occurs in a regime where |lambda| is smaller than of order unity.