A postantennal organ occurs in three different families in Collembola. In spite of considerable diversity in their outer structures, the organs manifest many similarities in their internal structure (pore system with pores of about 50-100 A diameter, a single sensory cell with two branching ciliary outer segments, reduction of the outer receptor lymph cavity and occurrence of an electron dense secretion around the dendrites). A tendency towards an enlargement of the outer surface of the organ and towards an incorporation of the sensory cell into the protocerebrum appears to be correlated with adaptations to a strict euedaphic life. An appropriate stimulus for the organ cannot be derived from the structure unequivocally, however, it could be hygro-, chemo- or thermosensitive. Perforated surface and number of pores in the postantennal organ correspond to the values found in insect sensory hairs.