Criteria of stochasticity (conditions of randomization of Larmor-rotation phases) are obtained for cyclotron heating in a non-uniform magnetic field. It is shown that randomization of phases is possible with the sufficiently large electric field of the wave. Criteria of stochasticity are given in the form suitable for the fields of the type of the magnetic trap. For the case of fulfilment of these conditions an equation is given which describes the variation in the distribution function and has the form of a Fokker-Planck equation.