Chlorophyll fluorescence has been used widely to detect the effect of herbicides in crops and weeds as it is a simple, sensitive and non-destructive method. Among the chlorophyll fluorescence parameters, maximum quantum efficiency, Fv/Fm, can be used to study the effects of herbicides as well as to monitor the development of herbicide resistance in weeds. In the present study, an attempt was made to assess the effectivity of sulfosulfuron, a sulfonylurea herbicide, in controlling Phalaris minor, a notorious weed of wheat crop in the Indo-Gangetic plains. Sulfosulfuron is very effective in controlling the isoproturon resistant population of P. minor. Differences in Fv/Fm values were observed among the control and treated plants within a week after treatment.