Spore, a tiny reproductive body that is typically unicellular. Spores are usually produced asexually. Many kinds of bacteria, protozoans, fungi, algae, and plants produce spores. Most of these organisms produce spores that, when the temperature and other environmental conditions are favorable, develop into new organisms. Seed-bearing plants produce two types of spores—microspores, which develop into pollen grains, and megaspores, which develop into embryo sacs, where eggs are fertilized.

Many bacteria produce endospores, spores that have a thick coating that protects them from extreme heat and dryness. Parasitic protozoans known as sporozoans produce sporozoites, spores that cause infectious diseases in animals. Unlike other spore-producing organisms, large fungi such as mushrooms normally produce spores sexually. Plants and multicelled algae go through a life cycle in which they alternately produce asexual spores and sex cells.

Individual spores are so small that they can be seen only with the aid of a microscope. Most of the spores produced by fungi, ferns, and mosses are powdery and are dispersed as they float in the air. The spores of most algae are waterborne and can swim by using a flagellum (tail).