Hypoxia is a condition of reduced oxygen concentration. Elite breath-hold divers (EBHD) during their habitual training are regularly exposed to acute transient hypoxia combined with skeletal muscle activation, interspersed with periods of normal breathing. Diving mammals and high-altitude natives who are also exposed to hypoxic conditions/environments have a higher capillary density, type I fibre distribution and myoglobin content in their most active skeletal muscles but a lower mitochondrial content than mammals and humans that live habitually in normoxic environments (1;2). Currently, little is known about the skeletal muscle characteristics of EBHD. Therefore, this study made the first investigations into the skeletal muscle phenotype of male EBHD. Resting percutaneous skeletal muscle biopsies were obtained from the m. vastus lateralis of 18 healthy anthropometry-matched male participants, of which 9 were EBHD (national team members) and 9 were non-divers (ND). Cryosections were stained with anti-lectin UEA-I to identify the capillaries and anti-myosin heavy chain type I to identify type I skeletal muscle fibres leaving type II fibres unstained. The fibre type stain was also co-incubated with either anti-myoglobin or anti-cytochrome C oxidase IV for determination of myoglobin and mitochondrial content, respectively. Images were captured and quantitated using immunofluorescence microscopy and analysed by ANOVAs. Data are means±SD. Capillary density was greater (19%) in the EBHD (605.1±87.1cap/mm2) compared to the ND group (490.2±98.9cap/mm2;p=0.019). For both groups the diffusion distance (R95) was lower in type I fibres (EBHD 27.59±3.1µm2; ND 31.16±3.8µm2, p<0.05) versus type II fibres (EBHD 33.5±4.4µm2; ND 37±6.1µm2), with a lower R95 for type I fibres in the EBHD versus ND (13%, p=0.045). Type I fibres had a higher myoglobin content (27%) than type II fibres in EBHD (p=0.002), whereas no fibre type differences were observed in the ND group (p=0.879). Type I fibre myoglobin content was significantly higher (27%) in EBHD than ND (p=0.025), but type II fibre myoglobin content was not different between groups. Type I fibres had a higher mitochondrial content (35%) than type II fibres in EBHD (p=0.015), but no fibre type differences were observed for ND (p=0.635) or between groups (p=1). Fibre type distribution (EBHD type I 56%±12% vs ND type I 55±13%) and mean fibre area (EBHD 5575±806µm2 vs ND 5163±804µm2) were not different between groups (p>0.05). This is the first study to report the skeletal muscle phenotype of male EBHD. The novel findings suggest a greater capillary density, improved diffusion distance and myoglobin content in the skeletal muscle of EBHD and these supply a mitochondrial content that is not significantly different to ND.