This study was performed in order to assess the effect of heating in pre-and post-rigor muscle of fed cod, wild cod and farmed salmon harvested at different times of the year. The structural changes in muscle samples pre- heated from 5 to 60°C were qualitatively evaluated using both light and transmission electron microscopy techniques. The microstructural changes are discussed in relation to the liquid loss measured by a low-speed centrifugation test. The heat-induced structural changes varied between the fish tested, reflecting different degrees of post mortem degradation prior to heating, the muscle-pH and species-specific structural properties. The fed fish, both cod and salmon, underwent the most severe structural degradation. This reflected both the low muscle pH and the more severe post mortem degradation observed in these fish prior to heating, compared with the wild cod. Heating caused extensive shrinkage of the myofibrils and hence, widened intermyofibrillar and extracellular spaces in both the fed cod and the salmon muscle. In the sample of wild cod muscle, the extracellular spaces were narrow and the myofibrils were closely packed. The difference in heat-induced liquid loss of the fed compared with the wild cod muscle coincides with their different structural features, as observed both by LM and TEM. The better liquid-holding properties of the salmon muscle than the cod muscle are attributed to the species-specific ultrastructural features as observed with TEM. In addition to the denser appearance of the salmon myofibres, it is suggested that both fat droplets and aggregated sarcoplasmic proteins filling the intermyofibrillar and extracellular spaces are important in preventing release of liquid upon heating.