The macrophyte distribution of Lake Alexandrina (737 m a.s.l.), a small, homothermous, South Canterbury high‐country lake, is closely related to depth. Shallow water communities, mostly dominated byIsoetes alpinusKirk, extend to a depth of about 4 m, but do not grow above 0.7 m because of strong wave action. Mid‐depth communities consist of a tallElodea canadensisMichx./Myriophyllum triphyllumOrchard community between 2 and 4 m at the shallow northern end of the lake, large areas ofChara fibrosavar.acanthopitys(A.Br). Zanev. at the southern end, and aMyriophyllum triphyllum/Nitella hookeriA.Br. community at 2–6 m on the steeply descending eastern and western sides of the lake. Pure stands ofElodea canadensismay replace the last community, particularly on the eastern side. A deep water characean community ofChara globularisThuill. andChara corallinaKl. ex Willd. extends from 4 m to 10–11 m with 100% cover;C. corallinacontinuing sparsely to 16 m. Maximum above‐ground biomass recorded for the summer 1982–83 was 1350 g m‐2(dry weight) forC. globularisand 1100 g m‐2forE. canadensis.