A mixture of essential oils as a growth promotor of a one year old gilthead seabream Sparus aurata (Linnaeus, 1758) The purpose of this study was to investigate the influence of a commercial preparation as growth promoter on one-year gilthead seabream Sparus aurata. The preparation is used as a growth promoter and antiparasitic agent in the poultry industry. For the purposes of the experiment, the investigated fish was taken from the same production cage, and 2 cages were formed with 1000 individuals in each. Cage 1 (K1) and cage 2 (K2) are fed with the same food. In food for K1 was added preparation and K2 served as a control group. The experiment lasted for 6 months and growth was monitored through a total of 13 measuring points to reveal the activity of growth enhancers. The results of this experiment have led to the conclusion that the preparation did not have a positive effect on growth because during the entire monitoring period there was a visible tendency for the fish from K2 (control group) to have a larger body length than the fish from K1, to which the preparation was added to the food. Also, the total mass of the body at the end of the experiment did not go into the preparation because the group fed with a preparation (K1) was statistically less mass than the control group (K2), which had a higher average mass on a continuous basis. Initial body weight was 60 g and the final 170 – 180 g.