A mesophile is an organism that grows best in moderate temperature, neither too hot nor too cold, typically between 20 and 45 °C with an optimal temperature near 37 °C, which is the normal temperature of the human body. Most organisms that are pathogenic to humans are mesophiles.

The habitats of these organisms include soil, the human body, animals, etc. The optimal temperature of pathogenic mesophiles, is 37 °C, the normal human body temperature. Mesophilic organisms have important uses in food preparation especially in cheese and yoghurt making and in beer and wine making.

Organisms that prefer cold environments are termed psychrophilic and those preferring hot temperatures are termed thermophilic.

See also: hyperthermophile