  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, $Q^2$, of less than
1 GeV$^2$ 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 $E_{T}^{\rm jet1}>14$ GeV and $E_{T}^{\rm
jet2}>11$ GeV and pseudorapidity requirements of $-1<\eta^{\rm jet1,2}<2.4$.
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.
