Mystacocarida is a species-poor group of minute crustaceans with unclear phylogenetic affinities. Previous studies have highlighted the putative “primitiveness” of several mystacocarid features, including the architecture of the nervous system. Recent studies on arthropod neuroarchitecture have provided a wealth of characters valuable for phylogenetic reconstructions. To permit and facilitate comparison with these data, we used immunohistochemical labeling (against acetylated α-tubulin, serotonin and FMRFamide) on the mystacocarid Derocheilocaris remanei, analyzing it with confocal laser-scanning microscopy and 3D reconstruction. The mystacocarid brain is fairly elongated, exhibiting a complicated stereotypic arrangement of neurite bundles. However, none of the applied markers provided evidence of structured neuropils such as a central body or olfactory glomeruli. A completely fused subesophageal ganglion is not present, all segmental soma clusters of the respective neuromeres still being delimitable. The distinct mandibular commissure comprises neurite bundles from more anterior regions, leading us to propose that it may have fused with an ancestral posterior tritocerebral commissure. The postcephalic ventral nervous system displays a typical ladder-like structure with separated ganglia which bears some resemblance to larval stages in other crustaceans. Ganglia and commissures are also present in the first three limbless “abdominal” segments, which casts doubt on the notion of a clear-cut distinction between thorax and abdomen. An unpaired longitudinal median neurite bundle is present and discussed as a potential tetraconate autapomorphy. Additionally, a paired latero-longitudinal neurite bundle extends along the trunk. It is connected to the intersegmental nerves and most likely fulfils neurohemal functions. We report the complete absence of serotonin-ir neurons in the ventral nervous system, which is a unique condition in arthropods and herein interpreted as a derived character. J. Morphol., 2010. © 2009 Wiley-Liss, Inc.