Amensalism is a biological interaction, a type of symbiosis, between two species in which one impedes or restricts the success of the other without being affected, positively or negatively, by the presence of the other. Usually this occurs when one organism exudes a chemical compound as part of its normal metabolism that is detrimental to another organism.

The bread mold Penicillium is a common example of this; penicillium secrete penicillin, which is a chemical that kills bacteria. A second example is the black walnut tree (Juglans nigra). Its roots secrete juglone, a chemical that harms or kills some species of neighboring plants.

In plants, amensalism is usually called allelopathy. Allelopathy research has often been controversial.Halsey, Richard W. 2004. In search of allelopathy: An eco-historical view of the investigation of chemical inhibition in California coastal sage scrub and chamise chaparral. The Journal of the Torrey Botanical Society 131: 343-367. ^* Many allelopathy studies are potentially misleading because they were conducted under artificial conditions without soil or with abnormally high levels of the allelochemicals.

Cornelius Muller's classic research on chaparral shrubs attributed the grass-free zones around the shrubs to volatile chemicals released by the shrubs. Muller's allelopathy hypothesis was disproved by Bruce Bartholomew, who fenced the shrubs to exclude animals and found that the grasses grew in the formerly grass-free zones. Small animals used the shrubs as cover, fed on the nearby grasses and created the bare zones.Halsey

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