In the mushroom bodies of the brain of the cricket Gryllus bimaculatus, the distribution of glutamate-like immunoreactivity is shown by using several immunocytochemical staining protocols and confocal and conventional microscopy. Glutamate-like staining of intrinsic cells of mushroom bodies (Kenyon cells), their axons and projections, is demonstrated for the first time. Two types of Kenyon cells constituting distinct, separated populations within the perikaryal layer and in prominent neuropilar subcompartments exhibit strong (type III cells) or medium (type II cells) glutamate-like immunoreactivity, whereas the small neurons of a central population (type I cells) lack staining above background. Type III Kenyon cells display a strong immunoreactivity similarly found in some giant neurons and in identified antennal motorneurons by using glutamate as an excitatory transmitter, indicating that also distinct populations of the Kenyon cells use glutamate as a putative transmitter. The pattern of glutamate-like immunoreactivity in the mushroom bodies and in other parts of the brain is different from gamma-aminobutyric acid (GABA)-like immunoreactivity (investigated for comparison). GABA-like immunostaining is particularly prominent in the mushroom body calyces where Kenyon cells have their dendritic branchings. Differences in glutamate-like immunostaining in Kenyon cell subpopulations, together with differences in their arborization and axonal projection patterns, indicate a functional diversity of these neurons.