Abstract: The p roductivity of Nostoc sphaeroides, an edible cyanobacterium found in paddy rice fields has declined in recent years Itmay relate to the increased app lication of agricultural herbicides1To assess the impact of glyphosate exposure (0.15, 0.30, 0.45 and 0.6 mmol/L Gly acid), the effects on colony size, dry biomass accumulation, chlorophyll a fluore scence (Fv/Fm) and chlorophyll abiosynthesis were investigated over an 8d period1All parameterswere significantly inhibited in a concentration used and time dependentway1After 2d exposure to >0.15 mmol/L Gly colonieswere app roximately 15% smaller than the controls1After 4d exposure, chlorophyll a content and Fv/Fm were affected by the highest concentration used (0.6 mmol/L Gly) 1By the 8d, chlorophyll biosynthesis and Fv/Fm were significant -ly inhibited by concentrations greater than 0.15 and 0.3 mmol/L Gly, respectively1The 8d relative growth rates (RGRs) calculated for dry biomass,were significantly affected by all glyphosate treatments, there was a 60% reduction at 0.15 mmol/L Gly and negative RGRs at higher concentrations indicate a loss of biomass1Exposure to 0.6 mmol/L Gly was lethal with loss of colony integrity,bleaching of p igments and no photosynthetic activity by 8d1These results are discussed in terms of the mechanisms of toxicity and the potential imp lications for the long term sustainability of the N1sphaeroides resource