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Background: Oxidation of myoglobin is responsible for the undesirable appearance and loss in acceptability of fish and fish products. The retardation of such a change by a modification of the surrounding atmosphere would be a means to maintain the quality of fish during the refrigerated storage., Results: The changes in oxymyoglobin and metmyoglobin from dark muscle of Eastern little tuna (Euthynnus affinis) as affected by different atmospheric systems (closed system, opened system and flushed oxygen system) were determined. A saturated oxygen atmosphere more likely weakened the haem-globin complex, especially as the exposure time increased. Autoxidation of the oxy form proceeded rapidly in the presence of oxygen with the concomitant formation of the met form. When the oxygen was excluded, oxidation of oxymyoglobin was retarded. With flushed oxygen and increasing exposure time, conformational changes of globin occurred, mainly associated with protein oxidation. Generally, oxymyoglobin was more susceptible to oxidation and conformational change than did metmyoglobin. After keeping the samples at 4 °C for 3 days, dark muscle of tuna fillet kept in vacuum packaging had a slight decrease in redness and it was still acceptable. The fillets stored in exposed air or packed in 100% O(2) atmosphere turned brown, most likely due to myoglobin oxidation., Conclusion: The oxygen level of the packaging atmosphere had a profound impact on myoglobin alteration, which was governed by the forms of myoglobin., (Copyright © 2011 Society of Chemical Industry.)