Adult grass shrimp were exposed to four concentrations (0.5, 1.0, 2.0 and 4.0 ppm) of hexavalent chromium for 38 days. At the end of the exposure period, over 50% of the surviving shrimp possessed cuticular lesions that had many of the gross characteristics of 'shell disease.' These lesions were usually associated with articulations of the appendages and abdomen. Furthermore, it was found that at increasing levels of chromium exposure, there was a proportionate increase in the loss of limbs such that nearly 50% of the limbs were lost in grass shrimp exposed to the highest test concentration of chromium. Histological and ultrastructural examination of numerous lesions demonstrated a range of degenerative features within the subcuticular epithelium that included cytoplasmic vacuolization, mitochondrial swelling, chromatin emargination, and the presence of unusual nuclear inclusions that appear to indicate direct chromium toxicity.