Economic Injury Level for Colorado Potato Beetle, Leptinotarsa decemlineata (Say), on ?Agria? Potatoes in Ardabil, IranAbstract : The Colorado potato beetle, Leptinotarsa decemlineata (Say), which had been a quarantine pest to Iran until 1984, has become the most injurious pest of potatoes in Ardabil plain at northwest of the country, where more than 25000 ha of irrigated land is allocated to potato production and more than 20% of the annual potato production of the country is produced. Uncontrolled populations of the first generation of the Colorado potato beetle may completely defoliate potato plant and destroy the crop. Potato growers in Ardabil region almost always apply insecticides to control the pest, yet little information on the economic benefits of these control measures are available. This study was carried out in years 2004 and 2005 in Ardabil. At each of two study years experiments were conducted in three different locations in Ardabil plain. At each location a randomized complete block experiment with four replications and five treatments were used. Treatments were densities of zero, 5, 10, 15 and 20 late (3 rd and 4 th instars) larvae per plant of the first generation that were caged on a potato plant. The purpose of this study was to determine the relationship between different densities of the first generation larvae of the Colorado potato beetle and the yield of the potato cultivar ?Agria? and to establish economic injury level for the pest. A linear relationship was observed between the insect density and the marketable yield in 2004 and 2005 at all three locations. The overall linear regression of two study years for three locations accounted for 93.39% of variation among the treatments. Economic injury levels of the pest were calculated as 5 and 6.458 late larvae per plant for ?Agria? cultivar in years 2004 and 2005 respectively. Therefore, based upon the results obtained in this research the control measure for the Colorado potato beetle is not recommended at pest densities of lower than 5 late larvae per plant , because the chemical control at lower densities is not only economically unfeasible but also by producing deleterious effect on natural enemies causes damage to the environment.