The Benguela upwelling in Namibia and South Africa experienced an ecosystem collapse after overfishing in the 1960s and 1970s, and as several industrially important species became scarce, others grew abundant. One of these was Sufflogobius bibarbatus, also known as the pelagic goby. The Benguela ocean floor has many areas with low levels of oxygen combined with high concentrations of H2S. These potentially lethal conditions are avoided by most species, but the pelagic goby spend daytime in this inhospitable mud. In April 2008 a the research vessel “G.O. Sars” left port in Namibia, with a goal of investigating exactly how and why the goby prefer to seek shelter in such areas. Cytochrome c oxidase (COX) is the fourth complex in the electron transport chain of mitochondria. It uses oxygen as the terminal electron acceptor during oxidative phosphorylation. Without oxygen oxidative phosphorylation stops and ATP production has to rely on anaerobic glycolysis. Accumulation of the end-product lactate is potentially deadly, and must be avoided. H2S binds to COX and inhibits the interaction with oxygen, thus stopping oxidative phosphorylation, making the organism functionally anoxic. In this thesis I have used respirometry to investigate the hypoxia-tolerance and H2S-tolerence of the pelagic goby. Finally I have done real-time RT-PCR experiments to examine the expression of COX subunits I-III during exposure to anoxia and/or, H2S. My findings indicate that the pelagic goby is exceptionally good at taking up oxygen in hypoxia, being able to maintain resting oxygen consumption down to a water oxygen level of 5.3 % of air saturation. It does not appear to have any special mechanism for tolerating H2S, asides from the fact that it can survive exposures to anoxia for hours. During anoxia it accumulates lactate and builds up on oxygen debt. In nature, this oxygen debt is most likely paid off during the nocturnal migration from the bottom to well-oxygenated pelagic waters, and could be a main reason for the diurnal migration pattern of the pelagic goby. The expression of COX subunits I-III does not appear to be affected by either anoxia or H2S exposure.