The mosquito ferns, genus Azolla, are a peculiar genus (the only genus in the family Azollaceae) of six species of aquatic ferns. They are extremely reduced in form and specialized, looking nothing like conventional ferns but greatly resembling a moss.

Mosquito ferns float on the surface of water with their roots floating in the water. They form a symbiotic relationship with a cyanobacterium, Anabaena azollae, which allows the plants to fix nitrogen from the air.

Because of their nitrogen-fixing capability, mosquito ferns have enabled an agricultural revolution in parts of southeast Asia. When rice paddies are flooded in the spring, they can be inoculated with the Azolla, which then quickly multiplies to cover the water. As the plants die, they contribute nitrogen to the rice plants, and as the rice paddy dries out, the Azolla all eventually dies, making an exceptional green fertilizer.

Mosquito ferns, however, are also serious weeds in many parts of the world, covering bodies of water so thickly that no water is exposed. This is where they derive their common name, from the belief that no mosquito can penetrate the coating of fern to lay its eggs in the water. Azolla is reputed to be able to grow so quickly that it can double its mass in three days under good conditions.

Most of the species can produce large amounts of anthocyanins in bright sunlight, producing an intense red color, so that the water surface may appear to be covered with a red carpet.

Mosquito ferns are safe to grow in cool temperate areas with prolonged freezing in winter, as they cannot overwinter in these conditions. They are often grown as an ornamental by water-garden hobbyists.