Bacteriological agar is a colloidal substance and a cell wall component, obtained from Rhodoyceae (marine algae) and Gelidium sp.Agarose combined with agaropectin forms this polysaccharide. It is also called as agar-agar and is a polymer of galactoside. This agar is soluble in boiling water but not in cold water.

Bacteriological agar is commonly used as a culture medium for microorganism. It is useful for fermentation process.