Climate change has led to an increase in CO2 absorption by the oceans. An increase in CO2 and a decrease in pH have been named ocean acidification. Recent studies have shown that not only do calcifiers suffer due to an increased solubility of calcium carbonate or other physical consequences, but fish experiences behavioral alterations. The aim of the present study was to investigate if the Three-spined Stickleback (Gasterosteus aculeatus) were affected by an elevated level of CO2.Effects were measured by conducting two behavioral experiments. One was to test if the fish were lateralizatised and the other was a test of exploratory behavior. Lateralization is a kind of right- or left handedness and decides the default action path in the relation to a stimulus. Lateralization was tested with a double T-chamber where the fish were gently forced to choose either a right of left path. The escape experiment was conducted to test exploratory behavior and boldness. The fish were put in a container in a tank with the possibility to escape. Results showed that lateralization was highly affected by experiment treatments. Fish kept at higher levels of CO2 lost their lateralization both at individual and population level. No time differences were observed in the escape experiment. The fact that an otherwise tolerant fish species showed behavioral disturbances could indicate that other teleost fish could be affected also.