The basic morphological organization of pigmentary effectors from the epi dermis and internal organs of the freshwater palaemonid shrimps Macrobrachium o!ftrsii and Macrobrachium heterochirus, the marine shrimps Pa!aemon affinis and Palaemon northropi, and the fiddler crab Uca rapax, was investigated by light and transmission electron microscopy. All pigmentary effectors (chromatosomes) examined comprise groups of 2â€"12 tightly bound, uninucleate cells (chromato phores), each possessing one or two cell extensions. Constituent chromatophores contained either a single or several pigment granule types of one or different colors. Desmosomes were noted between the membranes of adjacent chromatophores, al though a limiting basement membrane was only present surrounding chromato somes from the internal organs. Multinucleate or syncytial chromatophores were not encountered. Terminology currently used to describe crustacean pigmentary units is discussed in relation to the different levels of organization revealed.