In the United States the seasonal occurrence of the different stages of the common cattle grub, Hypoderma lineatum (De Vill.) and the northern cattle grub, H. bovis (L.), varies according to the latitude. To insure control of first-instar larvae with systemic insecticides, these compounds must be administered after heel fly activity has ceased and before larvae appear in the backs of cattle. During this period first-instar larvae of lineatum are located in the submucosal tissues of the esophagus. In any area, information on the abundance of larvae in these tissues would aid in determining the proper time of treatment with systemic insecticides.