Cholinergic neurons contribute to the directionally-selective properties of the crayfish Tan-1 neuron. This work sought to identify these cholinergic neurons in the optic lobe of crayfish and shrimp, using an antiserum to lobster brain choline acetyltransferase (ChAT; Brandon & Garrett, SFN 2002). Identification of histamine (His), the photoreceptor transmitter of many arthropod eyes, also is described here for the first time in these species. In both species, all photoreceptors (PR) were immunoreactive (IR) for ChAT. All regions of the retinular cells were ChAT-IR, with particularly strong labeling in their axon terminal regions within cartridges of the Lamina Ganglionaris (LG). In the shrimp LG, histamine-IR was observed in three tangential layers of punctate processes connected by radial fibers, forming a dense 3-dimensional network that surrounded the PR cartridges. His-IR cell bodies were present in the external chiasm. The crayfish LG contained no His-IR processes. PRs themselves were not His-IR in e...