Since the presence of the benthic, Macro-algal species Dichotomosiphon tuberosus was first identified in Lake Simcoe in 1978 by Ministry of the Environment Staff, three surveys have been conducted by Limnos Limited to document its distribution, standing crop and environmental significance to the lake. The data reported herein relate to the findings of the third field study conducted in August 1990, together with summaries of and comparisons with results previously reported. More specifically, the objectives for the 1990 field survey were to establish the current distribution within the lake, measure standing crop biomass and determine the tissue content of plant nutrients and selected metals.