The ratio between underwater quantum irradiance (q) and irradiance (E) for Jerlov's <1976> oceanic and coastal water types and for 11 Estonian and Finnish lakes was studied. This ratio was found to depend on the depth in the water body and the transparency of the water. The ratio q/E for the photosynthetically active radiation (PAR) region of the spectrum may differ from its value in air by up to 24%. The results of the present paper can be used to convert the underwater radiation data from units of W m-2 to μmol s-1 m-2 (or vice versa) and to estimate the errors caused by applying the air value of q/E instead of its real value in the water. ¿ 1998 American Geophysical Union