Structure of pseudoplacoid sensilla characteristic for Amphiesmenoptera by SEM and TEM methods was studied in the mushroom-like sensilla of Philopotamus montanus Don., a member of the primitive family Philopotamidae. These sensilla have been found different from the externally similar placoid sensilla in the presence of the socket-like depression and the sensillary stem rising from it. The expanded terminal part filled with the dendrites of the sensory neurons has numerous pores on its upper and lower surfaces. The sensillary dendrites have subterminal expansions filled with electron-transparent fluid, while their thin terminal parts have electron-dense central elements. A pseudoplacoid sensillum in the studied species has three sensory neurons. A comparison of the mushroom-like pseudoplacoid sensilla in Ph. montanus with chemoreceptive sensilla of other insects was performed.