Melt margarine in a large saucepan over low heat.

Add marshmallows to the melted margarine and stir until completely melted.

Continue cooking for 3 to 4 minutes, stirring constantly, until the marshmallow mixture is smooth.

Remove the saucepan from the heat.

Add Rice Krispies cereal to the melted marshmallow mixture.

Stir until the Rice Krispies are thoroughly coated with the marshmallow mixture.

Using a buttered spatula or wax paper, press the mixture evenly into a buttered 13 x 9 x 2-inch pan.

Allow the treats to cool completely.

Once cooled, cut the mixture into 2-inch squares and serve.