Karenia brevis is an autotrophic dinoflagellate responsible for many harmful algae bloom (HAB) events in the Gulf of Mexico. Behavior is an integral part of the life history of K. brevis, and swimming data is used in modeling bloom events. In this study, three strains of K. brevis, Apalachicola (APA), Manasota (MAN), and Jacksonville (JAX), are examined under a range of light intensities and temperatures that correspond to the viable range of the organism. In Part I, cell swimming speed is examined over a temperature range from 13∞C - 30∞C and swimming speed is examined with respect to an increasing stimulus light to consider the immediate effect of increasing light on swimming speed. Swimming speed remains fairly constant over all temperatures but the coldest. While examined under increasing light, swimming speed tends to increase. The increase in swimming speed with increases in light is similar to the response of Gyrodinium dorsum, but the increase in speed with Karenia brevis is only observable after increases in light intensity. In Part II, Electron Transfer Rate (ETR) and photosynthetic yield are examined for cultures acclimated to different temperatures. The two strains have opposite trends, APA has higher ETR at higher temperatures and MAN has higher ETR at lower temperatures. In another experiment, in order to examine the relationship between swimming capabilities, light exposure, and the cell's internal physiological state, swimming speed (measured with red light), photosynthetic yield, ETR, Adenosine triphosphate (ATP) concentration, and neutral lipid content are examined after 6 h incubations at 10 light intensities in the radial photosynthetron. ETR increases as light increases from low to higher light and swimming speed decreases. As light increases more, ETR decreases and swimming speed increases. With further increases in light, ETR and swimming speed decrease. Neutral lipids follow a pattern similar to ETR, only lipids peak after ETR at a light intensity that corresponds to the increase in swimming speed. The results, which suggest complex swimming-physiological relationships, generate questions to how cells partition energy. Swimming speed, yield, and ETR are examined over 12 h incubations as well - with sampling throughout the day. The patterns in these experiments are more difficult to interpret. In Chapter 2, cell surface aggregation patterns among K. brevis strains are examined. The strains show distinct and consistent surface patterns. There are no apparent differences in lipid content and organelle distribution in samples of the aggregates of the strains. Throughout this study, APA tends to swim the fastest when taped in the dark, whereas JAX tends to swim the slowest. Despite variation among the strains examined, the overall swimming speed of the species is in line with accepted values. Information from the three strains is combined to produce a species response to light and temperature ranges, and for comparison with physiological data.