Polyclonal antisera against allatostatin I(Dip-AST I), originally isolated from the cockroach Diploptera punctata, have been used to analyze both localization Dip- AST I -immunoreactive (Dip-AST I-IR) cell bodies and projection of their axons in the developing brains from the silkworm Bombyx mori. Dip-AST I-IR neurons were detected from all the brains investigated. During all developing periods, each brain has 20-80 labeled cells. Brains in most developing stages investigated contain a large number of immunoreactive cells, whereas relatively small number of labeled cells could be demonstrated in the 1st-instar-larval and 10-day-old pupal brains. These neurons have functions as neurosecretory cells, internueons and projection neurons in the brains. Median neurosecretory cells in the pars intercerebralis extend their axons to retrocerebral comlex by way of nervi corporis cardiaci (NCC) I, whereas lateral neurosecretory cells in the pars lateralis of protocerebrum project their axons into the retrocerebral complex through NCC II. Tritocerebral neurosecretory cells are also detected, but their axons could not be demonstrated.