Field vegetation surveys were used to evaluate some of the target and nontarget impacts associated with whole-lake applications of Sonar®*1A.S. aquatic herbicide (active ingredient, fluridone) applied to Michigan lakes between 1990 and 1996. The applications analyzed were made in the spring to control Eurasian watermilfoil (Myriophyllum spicatumL.) and/ or curly leaf pondweed (Potamogeton crispusL.). Dose-rate response curves for the control of individual aquatic plant species were constructed from plant species lists compiled before and after treatment. Plant species varied considerably in their response to Sonar. Eurasian watermilf oil, curly leaf pondweed, elodea (Ehdea canadensisMichaux.), naiads (Najas sp.), and coontail (Ceratophyllum demersum L.) were highly susceptible; bladderwort (Utricularia sp.) and water stargrass (Heteranthera dubia(Jacq.) MacM.) were very tolerant; and large-leaf pondweeds (Potamogeton sp.), wild celery (Vallisneria americanaMichaux.), narrow-leaf pondweeds (Potamogeton sp.), medium-leaf pondweeds (Potamogeton sp.), and flatstem pondweed (Potamogeton zosteriformisFern.) exhibited intermediate susceptibility. Sonar A.S. impacts on plant cover and diversity were highly dose dependent. Both cover and diversity usually declined during the year of treatment following applications at dose rates above 10 ppb (calculated on the volume of the top 10-ft layer of the lake). Cover and diversity typically recovered within 1 to 3 years following applications at these rates. Applications at dose rates of 10 ppb or less were typically much more selective. Impacts of low-dose rate treatments on total and native plant cover depended on the abundance of highly sensitive plant species, including Eurasian watermilfoil, curly leaf pondweed, elodea, coontail, naiad, and native watermilfoil. Where these species constituted a small part of the community, total native plant cover often increased following treatment. Low-dose rate (<10 ppb) Sonar applications are an effective method for controlling Eurasian watermilfoil and curly leaf pondweed with minimal impact on many native plant species.