The Caspian Sea as the greatest lake in the world has been faced with the different types of pollution. Among the different types of pollutants, petroleum (hydrocarbons) is one of the most important pollutants in the aquatic environment. Considering the economic and fisheries importance of the Caspian Sea, it is necessary to investigate the effect of such pollutant on valuable and endemic fish species. Since Caspian Kutum (Rutilus frisii) is a native and economic species of the Caspian Sea in the southern parts, the present study aimed to investigate the acute effects of water-soluble fraction of crude oil (WSF) on some physiological parameters of Caspian Kutum fingerlings at different times. Two experimental groups (WSF with LC50 of 0 and 50%) with three replicates for each one were considered in the experiment. Eighty fish (average weight of 0. 77 ± 0. 06 g) for each replicate were kept in an aquarium with the dimensions of 50 × 30 × 70 cm and exposed to WSF for 120 h. Then, 70 L WSF solution was added to each aquarium with the mentioned concentrations. Fifteen fish were sampled at 0 (before adding WSF), 6, 12, 24, 48, 72, 96 and 120 h after exposure to WSF for the measurement of lactate dehydrogenase activity and whole-body cortisol concentration. The results showed that the lactate dehydrogenase activity in fingerlings exposed to WSF (LC50=50%) was significantly higher than those kept in sea water (LC50=0%) during the sampling times (P<0. 05). There was a significant interaction between WSF concentration and time on the whole-body cortisol levels (P<0. 05). The results showed that crude oil had adverse effects on the physiology and welfare of Kutum fingerlings and the physiological responses were amplified in association with prolonged exposure to WSF.