Pultruded glass-fiber-reinforced vinyl ester composite coupons were subjected to four-point-bend environmental fatigue to study their long-term durability for infrastructure applications. Specimens were tested dry and while immersed in water and in solutions of water containing mass fractions of 5 and 10% NaCl salt. Some specimens were also preconditioned by soaking in water or salt solutions for five to six months without loading; the preconditioned specimens showed a fractional decrease of 5 to 13% in flexural strength compared to dry specimens. For specimens cyclically loaded at or above 45% of the average flexural strength of dry coupons, no change in fatigue life was observed for specimens tested while immersed in the fluids (with or without preconditioning) as compared to specimens tested dry. However, water and salt solutions are detrimental to the fatigue life of the material during long-term loading: at a cyclic load of 30% flexural strength, all specimens tested in air survived beyond 107 cycles, while all those tested under environmental fatigue did not. It is found that long-term environmental fatigue behavior is not controlled by the quantity of water absorbed; rather, it is governed by a combination of both load and fluid environment. However, a difference in fatigue life in the different fluid environments was not demonstrated. Microscopic examination revealed evidence of degraded fiber/matrix interphase, which is believed to be a controlling factor in the environmental performance of the glass composite.