Although outlawed in the European Union for health reasons, it is found mainly in Sardinia, Italy on the black market. Casu marzu literally means "rotten cheese" in Sardinian and is known colloquially as maggot cheese.

Derived from Pecorino, Casu marzu goes beyond typical fermentation to a stage most would consider decomposition, brought about by the digestive action of the larvae of the cheese fly Piophila casei. These larvae are deliberately introduced to the cheese, promoting an advanced level of fermentation and breaking down of the cheeses fats. The texture of the cheese becomes very soft, with some liquid (called lagrima, from the Sardinian for "tears") seeping out. The larvae themselves appear as translucent white worms, about 8 millimetres (0.3 in) long. When disturbed, the larvae can launch themselves for distances up to 15 centimetres (6 in). Some people clear the larvae from the cheese before consuming; others do not.