  The periodic Floer homology of a surface symplectomorphism, defined by the
first author and M. Thaddeus, is the homology of a chain complex which is
generated by certain unions of periodic orbits, and whose differential counts
certain embedded pseudoholomorphic curves in R cross the mapping torus. It is
conjectured to recover the Seiberg-Witten Floer homology of the mapping torus
for most spin-c structures, and is related to a variant of contact homology. In
this paper we compute the periodic Floer homology of some Dehn twists.
