Casu marzu (also called casu modde, casu cundhídu in Sardinian language, or in Italian formaggio marcio, "rotten cheese") is a traditional Sardinian sheep milk cheese, notable for being riddled with live insect larvae. It is found mainly in Sardinia, Italy.

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 cheese's 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.[1] 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 while others do not.