Lytechinus variegatus (31.29 ± 0.12 SE) were collected in October 2004 from Saint Joseph Bay, Florida (30°N, 85.5°W), and transported to the Texas AgriLife Research Mariculture Laboratory in Port Aransas, Texas. Sea urchins were held individually in 25 × 10 cm diameter cylindrical cages and fed semipurified formulated feeds containing 0.6, 1.1, or 2.4 mg selenium/kg feed (as sodium selenite) for 12 wk (n = 16 individuals per feed treatment, 15.59 ± 0.18 SE g initial wet weight, 31.29 ± 0.12 SE mm diameter; 32 ± 2 ppt salinity and 22 ± 1 C). Sea urchins were weighed every 4 wk and dissected at the end of the 12-wk study. High levels of dietary selenium (2.4 mg Se/kg feed) significantly decreased weight gain, test diameter, test wet weight, and dry ovary production compared to sea urchins fed the 0.6 and/or the 1.1 mg selenium/kg feed, suggesting that high levels of dietary selenium negatively affect sea urchin physiology. Observable pathologies in sea urchins fed high levels of dietary selenium included red coelomic fluid and pigmented organs, consistent with an inflammatory response. Additionally, selenium accumulated in the gut, gonad, and test at levels generally proportional to selenium in the feed. A selenium requirement is suggested but not established in this 12-wk study. These data suggest toxicity for high levels of dietary selenium in L. variegatus, resulting in a variety of growth inhibitions and pathologies. © by the World Aquaculture Society 2010.