The nudibranch mollusc Tritonia diomedea has been a useful model system for studies of how the brain controls behavior. However, no broad study of T. diomedea field behavior exists--an important deficit since laboratory behaviors may differ from what occurs in nature. Here we report analysis of time-lapse video of the slugs in their natural habitat to describe behaviors and their relationships to sensory cues. We found that movements relative to conspecifics, prey, and predators correlated with direction of water flow. These observations lead to three new navigational hypotheses: regardless of the actual heading to the target, T. diomedea crawls (1) upstream toward potential mates, (2) upstream toward food, and (3) downstream away from predators. We also describe both the behavior and its sensory context for feeding, escape swims, mating, and egg-laying, among other behaviors. Field behaviors were similar to published descriptions of laboratory behavior. However, the field observations add contextual detail, including preceding and subsequent behaviors and interactions with suites of habitat features not present in the laboratory. For example, the escape swim, previously studied as an isolated behavior in response to a single stimulus, appears to be affected by multiple sensory modalities and coordinated with several other behaviors. Our work will provide a basis for future neuroethological experimentation and also is the first step in the study of navigation in T. diomedea.