Mushrooms smell like fish because they contain trimethylamine, the same chemical compound responsible for the odor of seafood. Some species produce this compound naturally as part of their biology, while others develop it as they spoil. The reason behind the smell depends entirely on whether you’re dealing with a naturally pungent species or a package of button mushrooms past their prime.
The Chemical Behind the Smell
Trimethylamine (TMA) is a volatile compound that produces the characteristic smell of rotten fish, cooked crab, and shrimp. It belongs to a family of nitrogen-containing molecules called amines, which bacteria and fungi generate when they break down proteins and other nitrogen-rich organic matter. TMA is the same molecule that builds up on the surface of aging seafood, which is why the connection between mushrooms and fish strikes people so immediately.
Certain mushroom species synthesize trimethylamine on their own, without any spoilage involved. Others develop it when bacteria colonize the mushroom’s surface after harvest, degrading its tissues and releasing amines as byproducts. In both cases, the molecule is identical. Your nose can’t tell the difference between TMA from a perfectly healthy wild mushroom and TMA from a decomposing grocery store pack.
Species That Naturally Smell Like Fish
A handful of wild mushroom species are famous for their seafood-like aroma, and the best studied is the Crab Brittlegill (Russula xerampelina). Researchers analyzing the airborne chemicals rising from fresh fruiting bodies found that trimethylamine and a related compound called trimethylamine N-oxide were the only volatile substances the mushroom released. No other seafood-associated chemicals were present. The mycologist Rolf Singer noted as early as 1926 that this species smelled strongly of lobster or herring, and the connection has been confirmed repeatedly with modern lab equipment since then.
Another well-known example is the Fishy Milkcap (Lactifluus volemus), which develops a distinctive fishy odor after being picked. One field guide describes the smell as resembling “a dead shad, which anglers will tell you is probably the most malodorous freshwater fish.” The scent intensifies when the mushrooms are dried, concentrating the volatile compounds. Despite the off-putting aroma, both the Crab Brittlegill and the Fishy Milkcap are edible and commonly eaten. The smell doesn’t carry into the flavor in any significant way, though the Fishy Milkcap has a slightly granular texture that not everyone enjoys.
Why Some Species Evolved This Odor
Fungi use volatile chemicals to interact with the world around them, particularly with insects. Many mushroom species depend on flies and beetles to spread their spores, and strong odors are one of the primary tools they use to attract those visitors. Research on fungal volatiles has shown that certain species produce compounds specifically designed to lure insects closer, sometimes by mimicking the smell of food, decaying matter, or even potential mates. Once an insect lands on a mushroom and picks up spores on its body, it carries them to new locations when it flies away.
Amine-rich smells like trimethylamine are particularly effective at attracting flies, which are naturally drawn to protein-rich, decomposing organic material. For a mushroom that relies on fly traffic for spore dispersal, smelling like fish is a competitive advantage rather than a flaw.
When the Smell Means Spoilage
If you’re smelling something fishy from store-bought white button, cremini, or portobello mushrooms, that’s a different situation entirely. These species should have a mild, pleasant, earthy aroma when fresh. A fishy or ammonia-like smell is a sign that bacteria have started breaking down the mushroom’s tissues, producing trimethylamine and other amines in the process.
Fresh mushrooms should feel dry to the touch but not shriveled, and button mushrooms should be light in color. As they age, they darken, develop a slimy film, and begin producing off-odors. If your mushrooms smell like fish or ammonia, they’ve gone bad and should be discarded. The bacterial colonies responsible for those smells can also produce other compounds you don’t want to eat.
Reducing the Fishy Smell in Edible Species
For wild mushrooms that are naturally fishy-smelling but perfectly safe to eat, a few kitchen techniques can help. Acid neutralizes trimethylamine through a simple chemical reaction. A squeeze of lemon juice, a splash of vinegar, or a vinegar-based marinade will break down TMA and significantly reduce the odor. This is the same trick used on seafood for exactly the same reason.
Cooking with high heat also drives off volatile amines, since trimethylamine evaporates readily. Sautéing, roasting, or grilling fishy-smelling mushrooms will release much of the TMA into the air before the dish reaches your plate. Combining heat with an acidic ingredient, like deglazing a pan with white wine after sautéing, addresses the smell from both directions at once. Soaking mushrooms briefly in milk before cooking can also help, since the casein protein in milk binds to TMA and pulls it out of the food. When you drain the milk, the bound trimethylamine goes with it.