Grease 13 x 9 x 2-inch pan. In large saucepan over low heat, melt margarine; stir in cocoa and marshmallows.
Cook over low heat, stirring constantly, until marshmallows are melted and mixture is smooth and well blended. Continue cooking and stirring 1 minute. Remove from heat. Add cereal; stir until well coated.
With lightly greased spatula or wax paper, press mixture evenly into prepared pan. Cool completely. Cut into bars. Store, covered, at room temperature. Makes 2 dozen bars.