The dijet cross section in photoproduction has been measured with the ZEUS detector at HERA using an integrated luminosity of 38.6 pb^-1. The events were required to have a virtuality of the incoming photon, Q², of less than 1 GeV² and a photon-proton centre-of-mass energy in the range 134 < W_{\gamma p} < 277 GeV. Each event contains at least two jets satisfying transverse-energy requirements of E_{T}^{jet1}> 14 GeV and E_T^jet2 > 11 GeV and pseudorapidity requirements of -1<\eta ^jet1,2<2.4. The measurements are compared to next-to-leading-order QCD predictions. The data show particular sensitivity to the density of partons in the photon, allowing the validity of the current parameterisations to be tested.

PACS numbers: 13.60.Le