In a medium to large saucepan, melt butter over low heat.

Add the marshmallows and stir until completely melted.

Remove from heat.

Add Trix cereal and stir until cereal is well coated.

Prepare 13x9 pan by spraying with cooking spray or coating with butter.

Butter a your hands or use wax paper to press the mixture firmly into the pan.

Allow to cool.

Cut into bars.