  We construct a chiral lagrangian for pentaquark baryons assuming that the
recently found Theta^+ (1540) state belongs to an antidecuplet of SU(3) flavor
symmetry with J^P = (1/2)^(+-). We derive the Gell-Mann-Okubo formulae for the
antidecuplet baryon masses, and a possible mixing between the antidecuplet and
the pentaquark octet. Then we calculate the cross sections for pi^- p -> K^-
Theta^+ and gamma n -> K^- Theta^+ using our chiral lagrangian. The resulting
amplitudes respect the underlying chiral symmetry of QCD correctly. We also
describe how to include the light vector mesons in the chiral lagrangian.
