[en] The orbit-averaged kinetic equation suitable for studying collisional transport in an axisymmetric mirror machine is derived and reduced to a three-dimensional equation in phase space in the case of ν/sub c/ << ω/sub b/ << Ω where ν/sub c/ is the collision frequency, ω/sub b/ is the bounce frequency, and Ω is the gyrofrequency. We found that the gyrophase dependence of the distribution function, ignored in previous Fokker-Planck calculations, is responsible for the diffusion of guiding centers and should be retained when the Larmor radius is comparable to the plasma scale length. The complete Fokker-Planck equation for a square-well magnetic mirror is given as a special case to clarify the approximations and assumptions involved in the radial Fokker-Planck code of Futch. The quasilinear transport code of Matsuda and Berk is also discussed relative to the orbit-averaged Fokker-Planck equation