The first discovered protocetid, Protocetus atavus was described by Fraas 1904 based on a cranium and a number of associated vertebrae and ribs found in middle Lutetian Tethyan marine limestone from Gebel Mokattam near Cairo, Egypt.

[1] Protocetus are believed to have had a streamlined, whale-like body around 2.5 metres (8.2 ft) long, but was probably primitive in some respects.

[2] Many protocetids (like Maiacetus, Rodhocetus) possessed well developed innominates and hind limbs, often attached to the backbone with a sacrum.

[2] Despite previous proposals to the contrary, the current general consensus is that Protocetus and other protocetids lacked tail flukes.

Similarly, it retained sufficient olfactory apparatus to have a good sense of smell, although it probably relied more on its eyesight to find prey.