Characteristics of the force (P)-velocity (V) relationship were investigated before and after fatigue in isolated diaphragm muscle of two species: mouse (n = 12), in which diaphragm is composed almost exclusively of fast-twitch fatigue-resistant fiber type, and rat (n = 12), in which diaphragm contains both fatigue-sensitive and fatigue-resistant fiber types. Following fatigue protocol, the mechanical performance of both the contraction and relaxation phases were significantly depressed in the two species. Consequences of the fatigue procedure on P-V characteristics differed markedly according to the two species. In mouse diaphragm, fatigue trial did not change the G curvature of the P-V hyperbola, the maximum mechanical efficiency Effmax, or the normalized peak power output M2. Conversely, in rat diaphragm, fatigue resulted in higher G curvature (p < 0.001), higher Effmax (p < 0.001) and a lower M2 (p < 0.001) compared with control values. This suggests that fatigue improved the economy of force generation in mixed rat diaphragm and not in mouse. This is consistent with an increased proportion of slow fatigue-resistant fibers caused by fatigue of fast fatigue-sensitive fibers in rat diaphragm.