A case of Melanosis in a Holstein heifer is described in this report. Many of the visceral organs, diaphragm and skeletal muscles have got dark-brown to black and diffuse pigmentation in different diameters on their surfaces and cuts. Microscopically, brown to black coloured pigment containing cells were detected in the heart, lung, liver, spleen and muscle tissues. After depigmentation process, it was seen that the normal histological structure of the organ disappeared in these pigmented region. There were large, vesicular and macrophage-like cells with small nuclei. In immunohistochemical staining, HMB45 was strongly positive in many tissues. © 2007 The Author Journal compilation 2007 Blackwell Verlag.