Alum treatment of a shallow lake, with mean depth 2 m and area 137 ha, curtailed internal loading of P for at least one year. Mean summer total P and chl a decreased from 76 and 27 micrograms/l, respectively, in 1978 before treatment, to 29 and 14 micrograms/l, while mean summer Secchi transparency increased from 1.6 to 2.2 m and blue green algae were no longer dominant. Macrophyte biomass and distribution returned during the post-alum year, 1980-1981, to previous levels after a 1979 four-month lake level drawdown had reduced biomass by 84 percent. The improved transparency, resulting from the decrease in chl a following treatment, could encourage greater biomass of macrophytes in the deeper area of the lake. Recovery of macrophyte biomass to predrawdown levels may restore internal loading of P through enrichment of deep water sediments during winter dieback and decomposition.