The non- relativistic equntions of motion fo r a chnrgedrnpar t icle , an electron , in a wave of slowly varying amplitude arornexamined . The ',/ilv e , whistle r wave , is supposed to be propagatingrnat an arbitr ary but small angle to a uniform background magneticrnfield , in nn infinite collisionless plasma . Approximate solutionsrnf or t he equations of motion of particles in cyclotron resonancernwith the shistler wave are evaluated by method of ave r aging .rnAn expr ession for the period of trapped par t icles i n a lIpotentialrnwell" and an integral of motion in the space of phase differencernare der ived .rn,