Soluble carbohydrate and starch were examined in shoots of Ceratophyllum demersum, Elodea canadensis, Myriophyllum exatbescens, Potamogeton folkosw and P . zosteriformis in Shoal Lake (Manitoba-Ontario) during the 1985 season. A wide range of variation was found for the same species on any given day among different sampling stations a t the same depth. Correlations of each of total soluble carbohydrate and starch with wa,ter depth were inconsistent on any given day, although inverse correlations were more common. I n M . exalbescens, roots .consistently contained more voluble carbohydrate than shoots. The soluble carbohydrate fraction of all species contained glucose, fructose and sucrose. However C. demersum also contained Rignificant and relatively constant proportions of melibiose, raffinose and stachyose. Starch cont,ent decreased during the season in C. demersum but increased in P. foliosus and P. wsteriformis shoots. E. canadensis and M . exalbescens showed no significant seasonal trenda. Starch cxnprised 30 ... 85 O/ , , of the total nonstructural carbohydrate in the 5 species. Seasonal carbo-hydriLt,c: and Rtnrch concentrations were not significantly correlated with each other in any of the niacrophytes examined. The proportion of starch relative to the total nonstructurnl carbohydrat,e increa,sed during t.he season in P. folioszls and P . zosteriformis.