Natural zeolites have been increasingly used in terrestrial animal and freshwater fish diets as a growth and health promoter, but few studies have considered marine fish. Therefore, this study was planned to investigate the effects of dietary zeolite (clinoptilolite) incorporation on growth, feed and nutrient utilizations, whole body composition, waste excretions, intestinal morphology, fillet iron (Fe) and aluminum (Al) accumulations in juvenile gilthead sea bream (Sparus aurata). In the experiment, zeolite was included at 0 (Z0, control), 10 (Z10), 20 (Z20), 30 (Z30) and 40 g/kg (Z40) in a commercial sea bream diet and fed to fish with an initial weight of about 9.1 g for 10 weeks. Dietary zeolite treatments had a significant increasing effect on final weight (linear, P=0.046; quadratic, P=0.002) and specific growth rate (SGR) (linear, P=0.057; quadratic, P=0.010). Feed conversion efficiencies (FCE) of fish were linearly improved (linear, P=0.013) whereas PER increased both linearly (linear, P=0.033) and quadratically (quadratic, P=0.005). Supplemental zeolite did not affect body moisture or lipid but there was a trend for an increase (linear, P<0.10) in ash and a quadratic trend in protein (P<0.10) were recorded. Increasing dietary zeolite levels increased liver Fe levels and led to a quadratic affect on fillet Al levels (P<0.05) but did not change liver Al or fillet Fe levels (P>0.10). No differences in ADCs for dry matter, protein and lipid were detected but ADCs for energy had a quadratic trend (P=0.068). A trend for a decrease in total and dissolved nitrogen (N) losses was detected as the level of zeolite increased in the diet (quadratic, P=0.071 and 0.089 respectively). Anterior intestinal folds and gut length did not change with increasing zeolite levels but posterior intestinal folds decreased (linear, P=0.013; quadratic, P=0.023) and gut index had a quadratic trend (P=0.081). Optimum dietary inclusion level of zeolite was estimated as 27.1 g/kg based on the maximization of SGR, PER and FCE values. (C) 2014 Elsevier B.V. All rights reserved.