Microstructural characterization of a cast acetabulum of ASTM F-75 alloy has been carried out in order to clarify conflicting reports from the literature. The present investigation revealed that although sigma (sigma) and M23C6 carbide were the only secondary phases formed in the face centered cubic cobalt-base alpha matrix (Co-alpha), as identified by X-ray diffraction, the observed microstructure was quite complex. Scanning and transmission electron microscopy indicated the presence of coarse and fine lamellar cellular colonies, grain boundary film carbide, and different types of coarse blocky particles, including single-phase sigma, dual-phase sigma/M23C6, a binary eutectic comprised of sigma and Co-alpha phases, and a three-phase feature comprising the binary eutectic and solid state formed M23C6. The carbide has probably formed during cooling from casting due to sigma metastability. While it is proposed that the some lamellar cellular colonies were formed by discontinuous precipitation, it is not clear whether all lamellar structures present in the as-cast alloy occurred due to the same mechanism. The results obtained for the tensile properties are discussed in view of the observed microstructure.