Casu martzu is a traditional Sardinian cheese known for containing live insect larvae. The name "casu martzu" translates from the Sardinian language as "rotten cheese." It is made from sheep's milk and is considered a variation of pecorino cheese. The cheese is deliberately exposed to cheese fly (Piophila casei) larvae. The flies lay their eggs in cracks that form in the cheese. When the eggs hatch, the larvae digest the fats in the cheese, promoting fermentation and breaking down the cheese's texture to a soft, spreadable consistency.

Casu martzu has a strong, pungent flavor, and is often spread on flatbread such as Sardinian pane carasau. Traditionally, the cheese is consumed with the living larvae inside. Some locals remove the larvae before eating, while others consume it with the live larvae. The presence of moving larvae is considered by some as a sign of freshness. When the larvae die, it may indicate the cheese has gone bad.

The origins of casu martzu are not precisely documented, but it has existed for centuries as a rural Sardinian food. The production and consumption are intertwined with local shepherd traditions. It was generally made for family and local community consumption. In recent decades, concerns over hygiene and food safety led to it being declared illegal under European Union food hygiene regulations. Nonetheless, it remains available on the black market and is sometimes served at private gatherings and festivals.

Casu martzu has been recognized by Guinness World Records as the world’s most dangerous cheese. Despite health warnings, some Sardinians continue to view it as a delicacy linked to regional identity and heritage. The cheese has also sparked debates about food culture, legality, and tradition in modern Italy.