A study of muscles of the dystrophic mouse has failed to substantiate earlier claims that these muscles were especially resistant to fatigue in vitro or that fast muscles are preferentially damaged. It has been found that the fast muscle selected for previous studies is very often unable to withstand isolation in an organ bath if it is working, and both the difficulty in removing the normal gastrocnemius muscle intact and the need to trim it surgically contribute independently toward its deterioration in vitro. The smaller dystrophic gastrocnemius muscle is less liable to excision damage, is able to satisfy its resting metabolic needs in nutrient solution, and requires no damaging dissection, but is nevertheless unable to recover normally from fatigue. Using EDL and soleus muscles which are small enough to withstand isolation in vitro, no differences are found between fatigue patterns of normal and dystrophic specimens. Responses to rest, KCl, and 2 mM caffeine are also quite similar, and the only distinguishing biomechanical characteristic we have found in dystrophic mouse muscle is a weaker contraction and a longer total twitch time.