As an engineering thermoplastic acetal homopolymer is often used in the manufacture of parts subjected to fatigue. This article presents the results of flexural fatigue tests on acetal Delrin 550 under different environmental conditions, namely ambient air, forced-air ventilation at ambient temperature, or oil kept at a constant temperature at 40°C. The fatigue tests were performed on specimens cut from extruded sheets at a constant frequency of 30 Hz. Surface temperatures of the test specimens were measured either optically or electrically, depending on the environment. The test results show that both acetal stress and surface temperature of specimen, which are somehow related, vary considerably from one type of environment to another. However, results show that the fatigue life of the specimen is mainly governed by the amplitude of the initial stress and is almost independent of the environment, Another subject examined was the effect of oil on acetal's mechanical properties. It was found that prolonged contact with oil produced slight variations in tensile properties, but no significant effect on fatigue, life.