Herbicides pose a threat to aquatic ecosystems, especially to phototrophic organisms such as benthic algae. Benthic diatoms may be a valuable indicator of toxic impacts of herbicides in aquatic systems. Information on the sensitivity of a wide range of freshwater benthic diatom taxa to herbicides is required in order to develop a new biomonitoring tool that can determine herbicide toxicity in rivers. Unfortunately herbicide sensitivity data is only available for very few species as current methods of conducting algae toxicity tests on individual taxa are lengthy and costly. This study utilised the rapid toxicity approach to produce reliable sensitivity data for numerous diatom genera in a 48 hour exposure toxicity test. Natural benthic diatom communities were collected from rocks in-situ and placed directly into rapid toxicity tests. The relative sensitivities of the diatom genera to eight common herbicides (Atrazine, Simazine, Hexazinone, Tebuthiuron, Diuron, MCPA, Glyphosate, 2,4-D) were determined. This study identifies diatom taxa most at risk of herbicide toxicity within a field collected multi-species benthic diatom community. The most sensitive genera to were; Encyonema,Cymbella, Gomphonema, Ulnaria, whilst more tolerant genera were; Eunotia, Achnanthidium, and Navicula. These results are compared to field based observations of benthic diatom community changes relative to measured herbicide concentrations. The results of this project to date are assessed in terms of the development of a new biomonitoring index that uses benthic diatoms to provide a cost effective and ecologically relevant method for the detection and assessment of herbicide impacts in rivers of North Queensland.