Two field collections from Lake Simcoe of a hitherto unreported species of green algae, Dichotomosiphon tuberosus, suggested that a new biological influence on the lake ecosystem may have developed in recent years. As strategies were being developed for the control of evident eutrophication, a contract was issued to develop preliminary information on the areal distribution of the species, algal biomass and an assessment of potential effects on the chemical and biological environment of the lake. Surveys to assess algal distribution were conducted using a towed diver over 28 km of bottom. As growth appeared to be restricted to depths of 3 to 10 m, most of the survey work was confined to the east shore where the principal area of suitable depth was present. Dichotomosiphon distribution as predicted from diver surveys and subsequent dredge sampling was identified on a chart and estimated to cover 56 km2, and a total of 8% of the lake bottom. Where good growth was present, it formed a continuous dark green mat 3 to 10 cm in depth. Biomass sampling using three replicate 522.6 cm2 (9" x 9") Ekman dredge samples at each of 42 locations, indicated growth ranging from 0 to 410 g/m2 dry weight (d.w.) and a mean of 104 g/m2 d.w. for stations where the alga was present. Eleven tissue samples analyzed by the Ontario Ministry of the Environment laboratory reported a relatively low organic fraction of 22.0%, indicating some sediments remained in the washed samples. Mean tissue nutrient content calculated on an ash-free basis was 0.5% phosphorus, and 5.9% nitrogen. Total organic carbon content was 51.0% (ash-free).