Glyphosate-based herbicides (GBHs) are the most widely used herbicides in the USA and worldwide. Previous studies have documented tolerance to glyphosate by certain algae but data available are insufficient for generalizations. This study was partially funded by university funds in support of B. Dabney's MS thesis research. Results showed that glyphosate and GBHs stimulate growth the harmful alga, Prymnesium parvum, at environmentally relevant concentrations. Management of P. parvum blooms may therefore need to consider the potential contribution to bloom formation by glyphosate-contaminated runoff in areas where GBHs are being used or by the direct aquatic application of GBHs for the control of nuisance macrophytes.