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 ϕ with potential V(ϕ) and a barotropic perfect fluid. The cosmological dynamics crucially depends on the asymptotic behavior of the quantity λ=-M_pV_ϕ/V^3/2. If λ is a constant, which corresponds to an inverse square potential V(ϕ)∝ϕ^-2, there exists one stable critical point that gives an acceleration of the Universe at late times. When λ→0 asymptotically, we can have a viable dark energy scenario in which the system approaches an instantaneous critical point that dynamically changes with λ. If |λ| 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 |λ| is smaller than of order unity.