An electron microscopical study was made of the coeloconic and placoid sensilla on the antennae of the aphids Aphis pomi, Macrosiphum euphorbiae, Nasonovia ribis-nigri, and Pemphigus bursarius. Scanning electron microscopy revealed some variation in morphology which may be functionally important but is more likely to reflect the evolution of these species. The placoid sensilla were shown by tranwo or three neurons is surrounded by two ensheathing cells. The ciliary regions of the dendrites pass through a vacuole into a cavity between an outer and an inner cuticle where they may be connected to the dendritic branches although such connections were no seen. Small pores (8 nm diameter) partially penetrate the cuticle implying that these sensilla have an olfactory function. They are suggested to be important in host selection by alate aphids. The coeloconic sensilla are poreless pegs with nonsensory cuticular projections at their tips. The distal portions of their dendrites contain densely packed microtubules and the cellular arrangement of the sensila is similar to that of the placoid sensilla. It is suggest that they may function as thermoreceptors.