Mustard is one of the major oilseed crops grown all over India. The mustard aphid, Lipaphis erysimi (Kalt.), is one of the important pests in Terai region of West Bengal, which causes severe yield loss. The quantum of damage is determined by finding out the values ofthe economic injury level (EIL) and economic threshold level (ETL) respectively (Weersink et al., 1991).In fact, the knowledge of the value of the parameter, ETL helps to reduce the crop loss (and ensure less pesticide application), and as a sequel, the profit is increased. Contextually, substantial knowledge is required on the dynamics of the pest population in order to determine the density of the pest infestation (imbibed under the concept of economic injury level (EIL)) and the knowledge of the value of EIL is an important input which monitors the quantum of pest damage resulting in crop loss. This communication is devoted to the development of a systematic statistical method (not based on the economic parameters as in Weersink et al., 1991) for the determination of ETL (defined explicitly in the Introduction section) of the pest, aphid in case of the oilseed crop, mustard plant based on a real-life data set on the pest (aphid) incidence on mustard crop from a special type of experiment designed for this purpose.