The parthenogenetic marbled crayfish (Procambarus spec.) has recently been introduced as a new preparation for neuroethological studies. Since isogeneity apparently limits inter-individual variation, this otherwise typical decapod species may be especially valuable for circadian studies. Locomotor activity of isolated marbled crayfish and agonistic activity of small social groups maintain circadian rhythmicity in constant darkness. As potential signals of circadian time information, levels of 5HT, N-acetylserotonin and melatonin were determined in brains of marbled crayfish at different daytimes. However, location and structural organization of crustacean circadian pacemakers are still elusive. Immunocytochemical and backfill studies in the marbled crayfish revealed neural structures that may correspond to portions of circadian pacemaker systems in the insect optic lobe. Position and additional chemical contents in two pigment-dispersing hormone-expressing neuron groups resembled insect pigment-dispersing factor-expressing cells in the lamina and the accessory medulla, a neuropil discussed as center for integration of timing information. Here, we discuss new findings about the possible organization of the circadian system of the marbled crayfish in the light of current knowledge about circadian clocks in crustacea.