Fiber types can be differentiated by analyzing the specific myosin heavy chain (MyHC) isoforms expressed by each fiber’s phenotype. The two major skeletal muscle fiber types are type I (slow-twitch) and type II (fast-twitch). Skeletal muscle type II fibers can be further subdivided into types IIa (fast red) and IIb (fast white). In several mammalian species a IIx (IId) myosin heavy chain isoform have been also described. Fiber-type composition varies extensively between muscles and in accordance with the functional requirements of the muscle. Endurance capacity is correlated with high percentages of type I and type IIa fibers whereas sprint capacity is correlated with high percentages of type II fibers. Samples from the dorsomedial area of the Longissimus dorsi skeletal muscle were analysed in order to compare fiber type composition among different cetacean species with different dives behaviour. Samples were collected following a standard necropsy procedure and fixed in 10% buffered formalin. Two monoclonal Anti-Myosin (Skeletal-Slow and Skeletal-fast) antibodies have been used for the localization of slow (Type I fibers) and fast (Type II) MyHC by using the immunohistochemical technique. Previous studies based in the presence of intramyocellular lipids in cetacean skeletal muscles and these new results will be presented and discussed in relationship with their behaviour.