any substance poisonous to an organism. The term is sometimes restricted to poisons spontaneously produced by living organisms (biotoxins). Besides the poisons produced by such microorganisms as bacteria, dinoflagellates, and algae, there are toxins from fungi (mycotoxins), higher plants (phytotoxins), and animals (zootoxins). The name phytotoxin may also refer to a substance, regardless of origin, poisonous to plants.

Some biotoxins appear to be of little use to the organisms producing them but may play unknown roles in the organisms' metabolism, or they may be waste products. Many others, however, confer obvious advantages on their producers, such as depressing the growth of competitors or preventing predation. Many of the thousands of known phytotoxins are known to protect their producers against certain animals, especially insects. Similar defensive secretions are numerous among animals and may be either widely distributed among the tissues or concentrated in certain tissues, in which case there is often some sort of delivery system, such as spines or fangs.

Numerous animals (e.g., spiders and some snakes) have evolved poisons as aids in securing prey and often use the same poison for defense.

Many normally edible fishes and shellfishes become poisonous after feeding on toxic plants or algae. When a toxic fish or shellfish is eaten, the poison attacks the consumer's nervous system and causes a sometimes fatal condition called ciguatera. See poison.