A bicirculant is a graph admitting an automorphism with two cycles of equal length in its cycle decomposition. A graph is said to be arc-transitive if its automorphism group acts transitively on the set of its arcs. All cubic and tetravalent arc-transitive bicirculants are known, and this paper gives a complete classification of connected pentavalent arc-transitive bicirculants. In particular, it is shown that, with the exception of seven particular graphs, a connected pentavalent bicirculant is arc-transitive if and only if it is isomorphic to a Cayley graph Cay(D 2n , {b, ba, ba r +1 , ba r 2 +r +1 , ba r 3 +r 2 +r +1 }) on the dihedral group D 2n = a, b | a n = b 2 = baba = 1 , where r ∈ Z * n such that r 4 + r 3 + r 2 + r + 1 ≡ 0 (mod n).

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