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Abstract. The mangrove mysid Mysidium columbine (Zimmer) occurs in aggregations of tens to millions of individuals. Few indepth studies on the aggregative behavior of this species have examined structural composition, response to light intensity changes, and its ability to reduce prcdation. Snorkel and SCUBA observations, as well as aquarium experimentation on shoals/ schools of this species in shallow waters around a mangrove archipelago were made at irregular intervals to determine their structure, dynamics, and influence on prcdation. In summer, aggregations occurred as large cylindrical shoals, in winter as small ovoid schools. In daylight, shoals/schools were very sensitive to changes in light intensity. At night the smallest juveniles formed compact swarms, while larger individuals became solitary. At sunrise a definite rc‐shoaling behavior existed. Shoals/schools were stratified by life stages. A hypothesis stating that conspecific schools in winter did not mix was tested. Mysidium shoals/schools seemed to discourage prcdation by fishes.