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The quality of the surface water of Gotland (Sweden) streams was good, and did not limit sea trout egg-to-fry (ETF) survival. The oxygen concentration of the interstitial water was positively correlated to the permeability of the stream bed, and to the geometric mean diameter of the substratum. When the oxygen at 15 cm inside the stream bed was undiminished from surface values, the permeability of the stream bed was at least 6000 cm h 1 and the geometric mean diameter of the substratum at least 15·0 mm. When the interstitial oxygen concentration increased, the interstitial ammonium concentration decreased, the NO 2 concentration remained stable and the NO 3 concentration increased. The ETF survival up to hatching was highly dependent upon the oxygen concentration of the interstitial water. For survival from hatching to emergence, the geometric mean diameter of the substratum was the most important factor. ETF survival >50% was observed where the interstitial oxygen concentration averaged at least 10 mg l 1 , the substratum permeability at least 2000 cm h 1 and the substratum geometric mean diameter at least 15 mm. Other factors, such as overdigging and drying out of the spawning grounds could increase the ETF mortality. 1996 The Fisheries Society of the British Isles