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Studies have demonstrated that a combination of mental and physical challenge can elicit exacerbated state anxiety, effort sense, and cortisol responses above that of a single stimulus. However, an analysis of the effects of aerobic fitness on the responses of cortisol to concurrent mental and physical stress between below average and above average fitness individuals has not been conducted. This study examined the effects of a combination of acute mental challenges and physical stress on psychological and cortisol responses between eight individuals of below average fitness (low fit (LF), V˙O2max = 36.58 ± 3.36 mL·kg−1·min−1) and eight individuals of above average fitness (high fit (HF), V˙O2max = 51.18 ± 2.09 mL·kg−1·min−1).All participants completed two experimental conditions. An exercise-alone condition (EAC) consisted of cycling at 60% V˙O2max for 37 min, and a dual-challenge condition (DCC) included concurrent participation in a mental challenge for 20 min while cycling.The DCC resulted in increases in state anxiety (P = 0.018), perceived overall workload (P = 0.001), and exacerbated cortisol responses (P = 0.04). Furthermore, LF participants had a greater overall cortisol response in the DCC compared with the EAC (DCC = 346.83 ± 226.92; EAC = −267.46 ± 132.32; t7 = 2.49, P = 0.04), whereas HF participants demonstrated no difference between conditions (DCC = 38.91 ± 147.01; EAC = −324.60 ± 182.78; t7 = 1.68, P = 0.14).LF individuals seem to demonstrate unnecessary and unfavorable responses to the DCC compared with HF individuals, particularly concerning cortisol. The exacerbated cortisol responses in LF individuals have implications for harmful consequences such as increased risk of cardiovascular disease.