The grass shrimp Palaemonetes pugio, a common inhabitant of brackish water marshes, is often used in experimental studies. To support studies concerned with the behavior of P. pugio in controlled laboratory conditions, a behavior catalog (or ethogram) was developed. Twelve grass shrimp activities, six moving and six stationary, were observed and described. The most frequently observed activities included hovering in the water column, walking and three types of swimming. Activities observed less often included backward thrusting, jabbing with the rostrum tip and taking cover. The value of the ethogram was assessed using a predator-prey experiment in which the frequency of grass shrimp behaviors in response the predatory fish Fundulus heteroclitus was examined. In the presence of a fish, grass shrimp decreased the frequency of swimming and increased the frequency of covered and motionless behaviors. We also noted a significant increase in the occurrence of two active antipredator defenses: backward thrust and jabbing with the rostrum tip. Backward thrust quickly propelled shrimp away from a fish. Jabbing with the rostrum tip was directed toward an advancing fish and usually caused the fish to swim off in the opposite direction. These observations provide clues as to the levels and types of interactions that occur between the grass shrimp and their fish predators in the wild. The catalog of recorded activities may be of value in future studies designed to examine the effect of different experimental conditions on grass shrimp behavior.