Harmful algal bloom (HAB) events are increasing in severity and frequency worldwide, and are known to severely impact fish populations. Impacts of HABs on fish, as well as other organisms, occur through toxic and physical stress. Behavioral and central nervous system (CNS) alterations can have direct consequences to the fitness and survival of individuals and populations. This study investigated and characterized alterations in social and swimming behaviors and brain activity in mummichog (Fundulus heteroclitus) exposed to HAB stressors. The mummichog is an ecologically important estuarine fish species exposed to a variety of HAB events in the wild. A behavioral analysis system was developed to study swimming and social behavior of fish and an immunohistochemistry technique was used to investigate alterations in neuronal activity as evidenced by c-Fos protein expression. HAB stressors included excitatory (domoic acid, brevetoxin) and inhibitory (saxitoxin) neurotoxic agents as well as direct exposures to the dinoflagellate Pfiesteria shumwayae and the diatom Chaetoceros concavicornis. P. shumwayae and C. concavicornis are HAB species that are known to induce mortality through physical trauma to fish. Brevetoxin exposure increased swimming and social behaviors whereas saxitoxin decreased these behaviors. The effects of saxitoxin on swimming and social behaviors were consistent with exposure to a fish anesthetic, MS-222. Similarly, it was found, through c-Fos expression, that the excitatory HAB neurotoxins brevetoxin and domoic acid, increased neuronal activity while saxitoxin decreased activity. Exposure to P. shumwayae and C. concavicornis, resulted in significant dose related increases in neuronal activity. Stressor-specific neuronal activity was greatest in the optic lobe, but was also found in the telencephalon with physical stressors increasing activity greater than chemical stressors. Results demonstrate that sublethal exposures to HAB neurotoxins can alter swimming and social behavior in mummichog and exposures to both neurotoxins and algae can alter neuronal activity. Alterations in brain activity, and knowledge of specific regions within the brain activated during stress, can provide insights into the control of fish behavior. Ultimately, HAB exposure related changes in neuronal signaling may alter behaviors, resulting in individual and population level alterations during HAB events.