Mesosaprobic organisms are organisms that develop in waters moderately polluted with organic matter. A distinction is made between α-mesosaprobic and β-mesosaprobic organisms. The former are able to develop under oxygen deficiency (e.g. microalgae, rotifers), the latter - under sufficiently high free oxygen content (sponges, bryozoans, etc.).

Mesosaprobic organisms (from Meso... and Greek saprós - rotten, bíos - life), plant and animal organisms living in water bodies (or parts of water bodies) moderately polluted with organic matter. Such waters (unlike heavily polluted waters inhabited by Polysaprobes and lightly polluted waters inhabited by oligosaprobes) contain free oxygen, oxidation products such as nitrates and nitrites (along with ammonia and weakly oxidized nitrogenous compounds such as amino acids and amines), but no undecomposed proteins. Mesosaprobic organisms contribute to biological self-purification of water bodies; some of them, developing in mass quantities, serve as an indicator (bioindicator) of water quality.