Copper is an essential micronutrient in the diets of animals. It is a component of many enzymes involved in energy production, participates in immune function, and protects cells from free radicals. However, excessive levels in the diet can be toxic. Small (∼13 g wet weight) Lytechinus variegatus were fed formulated feeds with 12, 36, or 114 mg Cu/kg for 12 wk (levels based on established dietary levels for other marine invertebrates, supplemented as CuSO₄·5H₂O). Under these experimental conditions, wet weights of individuals fed a 36-mg Cu/kg diet were slightly higher (43.2 ± 1.2 g (SEM); P = 0.069) than those fed a 12mg Cu/kg and 114-mg Cu/kg diet (39.9 ± 1.2 and 40.3 ± 1.7 g wet weight, respectively). Ovary and gut wet weights were significantly lower (P < 0.003) in the 114-mg Cu/kg diet than the 12-mg Cu/kg and 36-mg Cu/kg diets (7.24 ± 0.75 g, 8.11 ± 0.55 g, and 4.99 ± 0.32 g ovary wet weight and 0.97 ± 0.04 g, 1.07 ± 0.06 g, and 0.83 ± 0.04 g gut wet weight for the 12-, 36-, and 114-mg Cu/kg diets, respectively). Mature gamete formation in ovary and testis was inversely correlated with dietary copper level. Acini from the ovaries and testis of urchins in the 36-mg Cu/kg and 114-mg Cu/kg diet treatments had a greater area occupied by nutrient phagocytes than urchins on the 12-mg Cu/kg diet. In diets containing low dietary copper (12 mg Cu/kg), survivorship decreased from 100% to 87%. These data suggest that dietary copper is essential for normal physiological function but can be detrimental for certain physiological processes at high levels. This information will help in the development of formulated feeds for sea urchin aquaculture.