Plankton provides an essential foundation for life on Earth, supplying most of the breathable oxygen, carbon sequestration, and larvae nutrition. Toxic chemicals introduced into the environment pose a potential danger to plankton and the ecosystem. Traditional plankton chemical toxicity assays measure the concentration required to cause death. Sublethal doses that affect plankton activities like foraging and escaping predators can have a cascading impact on ecosystems. We have developed a high-throughput device to measure the sublethal effect of chemicals on the plankton rate of movement. The device automatically creates a series of chemical dilutions, subjects each concentration to a group of plankton, and calculates the average group movement speed. We tested the device on groups of Stentor coeruleus (n=3 to 26, mean=10) with acetic acid dilutions (43 to 25,000 ppm) and measured a declining trend in average speed with increasing sublethal concentrations (170 to 502 ppm).