The use of medium pressure UV systems to control dreissenid mussel settlement in industrial cooling water systems is a desirable alternative to chemical treatments. This paper summarizes two experiments, carried out over two years at Davis Dam, using a proprietary medium pressure UV system. The first experiment tested veliger settlement after exposure to doses of 50, 40 and 20 mJ/cm². The second experiment tested settlement after exposure to 100 mJ/cm2 , and examined veligers behavior and direct mortality post exposure to UV doses of 100, 50, 40 and 20 mJ/cm2 .