The aim of this study was to investigate whether Hirsutella sinensis mycelium (HSM) has any antifatigue effect, using a forced swimming model in rats. Forty rats were randomly divided into five groups, each containing eight animals. The control group received 2 ml/kg body weight of distilled water and a positive control group was administered 1.13 ml/kg Quaker Essence of Chicken. The treated swimming groups were administered HSM powder manufactured by Chang Gung Biotechnology Corporation, Ltd., at doses of 63 mg/kg, 189 mg/kg or 378 mg/kg body weight/day, respectively for a period of six weeks. The above experiment was repeated with another 40 rats but for a period of eight weeks. At the end of the experiments, rats were placed in a swimming apparatus and the total swimming time until exhaustion was recorded. Pre-/post-exercise concentrations of serum urea nitrogen (BUN) and lactic acid were also determined. There were no deaths during the study. Physical and behavioral examinations did not reveal any treatment-related adverse effects after dosing. Changes in lactate levels were dose-dependent for the 8- but not the 6-week treatment. BUN levels were more affected by the 8-week treatment of HSM but not significantly altered in the 6-week treatment groups. The 8-week treatment groups showed a significant increase in swimming time to exhaustion compared to the control groups, which was not dose-dependent. For the 6-week treatment, only the middle and high doses increased swimming time to exhaustion. Conjugated diene contents were significantly higher in rats treated at any HSM dose for 8-weeks than the control groups. Swimming did not alter levels of liver glycogen when compared to the control sub-groups. Results of this study demonstrate that HSM improves physical endurance, which may be beneficial in treating conditions where fatigue is a factor and other antifatigue treatments are contraindicated.