Representative hydrophyte samples were taken from comparative transects in four bodies of waters in southeastern Michigan. A simple laboratory technique was employed to determine quantitatively the amount of monocarbonate marl deposit present per gram of plant material (dry weight). Among the areas investigated, differences in marl encrustation were slight for like species under similar environmental conditions. Considerable variation in deposition was found among different species. Among emergent and floating hydrophytes, capable of direct utilization of atmospheric carbon dioxide, encrustation on submerged portions was highly variable and proportional to the extent of development of attached periphyton. Among submerged species deposits are considerably greater and variations are correlated with several factors, namely, morphological variations and differential bicarbonate utilization. Intraspecific variations were found also, with encrustation increasing with increasing depth on the individual hydrophyte. Although mechanical removal of the brittle deposits induced by surface wave action is the primary factor decreasing surface encrustation, other factors may be involved. Agencies of marl formation are discussed, including induced precipitation by bacteria, phytoplankton, periphyton, and macrophytes, as well as direct physical precipitation.