association between organisms of two different species in which one is inhibited or destroyed and the other is unaffected. There are two basic modes: competition (q.v.), in which a larger or stronger organism excludes a smaller or weaker one from living space or deprives it of food, and antibiosis, in which one organism is unaffected but the other is damaged or killed by a chemical secretion.

The classic demonstration of antibiosis is the destructive effect that the bread mold Penicillium has upon certain bacteria; the secretion, known as penicillin, has become a potent medicine in combating bacterial infections. Some higher plants secrete substances that inhibit the growth of—or kill outright—nearby competing plants. An example is the black walnut (Juglans nigra), which secretes juglone, a substance that destroys many herbaceous plants within its root zone.