Abstract A study of neurosecretory cells and corpus allatum of the 3rd instar nymph of Macrosiphum euphorbiae was undertaken by light and electron microscopy. Neurosecretory cells are referred to as group I cells. There are 3 or 4 of these cells on each side on the anterior face of the protocerebrum. Two cell types, which differ in size and ultrastructure, are distinguished in this group. Cells of the first type are characterized by a highly developed rough endoplasmic reticulum and many Golgi complexes, which include dense bodies. In the second cell type, these 2 cytoplasmic elements are scarce. As a consequence, these latter cells are taken to be less active than type 1 cells. However, further observations would be required to document this conclusion. Corpus allatum contains about 20 similar cells, which have very irregular shape and highly convoluted cell boundaries. Rough-surfaced endoplasmic reticulum is generally scarce, but free ribosomes are numerous; large aggregates of smooth membranes are also sometimes observed. Axons with secretory material are frequently seen between cells. Nerve connection with an allate cell is shown.