Two different polymeric materials were evaluated to determine their respective fatigue endurance behavior. Both materials were tested separately in laboratory air and in flowing water. (Water was selected as a suitable liquid test environment since neither polymer was adversely affected by its presence.) Polymer A showed similar S-N plots in the two environments whereas Polymer B revealed remarkably different results. Speculate as to which environment was associated with the superior fatigue response in Polymer B and characterize the structure and mechanical response of Polymers A and B