An aquatic organism growth promotor, containing, as an effective ingredient, a low molecular weight lignin having a molecular weight peak in a molecular weight range of 4,000 to 9,500 and/or a high molecular weight lignin having a molecular weight peak in a molecular weight range of 10,000 to 40,000, wherein the molecular weight peak is measured at a wavelength of 254 nm by GPC molecular weight analysis using an UV detector.
