For non-essential trace elements, the concept of a “critical body residue” or CBR has been explored over the years as a means of predicting whether or not the element is likely to exert deleterious effects on the host organism. However, aquatic organisms tend to accumulate metals in different forms, some of which remain internally bioavailable whereas others are detoxified; measures of total bioaccumulated metal ignore this phenomenon of subcellular metal partitioning. If the ratio of bioavailable to detoxified metal remains constant within the organism as metal exposure increases, then use of the CBR concept is appropriate. However, if the subcellular partitioning of the metal varies with metal exposure, then the critical body concentration approach no longer applies. Over the past 15+ years, we have investigated how aquatic organisms cope when exposed to non-essential metals such as silver, cadmium, nickel and lead. These metals normally enter living cells by facilitated transport, “fooling” the transmembrane transporters that are responsible for the influx of essential metals. Once inside the cell, the non-essential metals may be detoxified, for example by complexation with metallothionein or by sequestration in metal-containing granules. If detoxification is incomplete, these metals will be available to bind to metal-sensitive sites within the cell, such as enzymes or nucleic acids, potentially leading to deleterious effects. To determine whether metal detoxification has been effective or not, we have employed a two-pronged approach. The first step involves gentle cell rupture followed by differential centrifugation and heat denaturation, yielding five or more fractions: cellular debris; metal-rich granules; organelles; cytosol; heat-denatured cytosolic proteins (HDP); heat-stable cytosolic proteins (HSP). The HSP and granule-like fractions are normally considered to represent detoxified metal, whereas the HDP and organelle (e.g., mitochondria) fractions are classified as metal-sensitive fractions. The second step calls for the chromatographic separation of metal-ligand complexes present in the operationally-defined cytosol or the HSP fraction, to probe their chemical identity. This dual separation scheme has been used to compare the partitioning of different metals in a given species or of a specific metal across different species. In this communication, we compare the subcellular behaviour of metals in unicellular algae, an insect larva, a bivalve and several fish species. The algae were exposed under controlled conditions in the laboratory, whereas the animals were all collected in the field from contaminated sites representing metal contamination gradients. In all the field-collected animals, we were able to document a metal detoxification response involving the induction of metallothionein-like peptides (MTLP). Less frequently, the non-essential metal was sequestered in the granule-like fraction. The results show predictable differences between “soft” and “hard” cations, but for a given metal they also show inter-organ and subtle inter-species differences in the extent of metal detoxification. The implications of these findings for the application of the critical body residue approach will be discussed.