Their website at http://www.mcdonalds.com/us/en/food/product_nutrition.chickenfish.5926.filetofish.html says:
FISH FILET PATTY
Ingredients: Pollock, Wheat Flour, Water, Modified Food Starch, Contains 2% or Less: Yellow Corn Flour, Bleached Wheat Flour, Salt, Whey, Dextrose, Dried Yeast, Sugar, Cellulose Gum, Colored with Paprika and Turmeric Extract, Spice Extractives.