Heavy metals accumulated by aquatic crustaceans in environmental studies are normally investigated using the whole body burden, with little regard paid to uptake in different tissues, to potential gender of life stage differences, or to the influence of nutrition on the test organism. This is likely to give erroneous conclusions for a dose-response relationship within the toxicity test and potentially lead to wrong conclusions for the ecological risks of metals where species may have higher sensitivities with gender and life stage than indicated or that functionally metals may be sequestered into parts of the body so are not bioavailable. This could lead to under-estimation or over-estimation of the toxicity of metals, respectively, inaccuracy of metal budget calculations and evaluation of trophic transfers of metals. This study evaluated the influences of life stage, gender, and a priori nutritional state in the uptake of the metals zinc (an essential micro-nutrient; Zn) and cadmium (a non-essential element; Cd) in the amphipod Echinogammarus marinus. The study showed that life stage, and nutritional stage did significantly influence the uptake and bioaccumulation for both metals, but only Cd showed differential uptake and bioaccumulation with gender. In addition, it was concluded that there was a significant uptake and accumulation of both metals within the exoskeleton of the amphipods, which though adding to the full body burden would add little to toxicity through lack of bioavailability. These results showed that care should be taken when interpreting results from tests normally preformed on such test organisms.