The colonial peritrich Zoottiamnium sp. is a common ectocommensal of shi'imp. Heavy branchial infestations of this protozoan can cause mass moi'talities of commercially important penaeid species in shrimp culture. In an effort to better understand the morphology of these organisms, the ciliates and their attachment to gills and pereiopods of Penaeus aztecus and P. setiferus were examined by transmission and scanning electron microscopy. Each zooid contains peristomial rows of cilia, numerous food vacuoles, a large macronucleus, one or more contractile vacuoles, freeribosomes, abundant mitochondria, and rough endoplasmic reticulum. The contractile spasmonehie is surrounded by striated tubules that appear to develop from scopular cilia. The pellicular surface is crenated and has numerous pores. The base of the ciliate's stalk forms a circular disc that fuses with the epicuticle but does not penetrate the underlying cuticle or epithelium. Intertwining microflbrils and osmiophilic particles in the stalk accumulate along the base and merge with the cuticular surface coat. This mode of attachment inflicts no damage to the host tissue and elicits no appreciable pathological response. The shrimp apparently suffocate when dense colonies reduce the respiratory surface area.