Behavioral measurements of fragile aquatic organisms require specialized in situ techniques.We developed an in situ brightfield camera set-up for use during SCUBA diving in aquatic ecosystems.The system uses brightfield illumination with collimated light and an underwater camera to highlight morphological details, body motion and interactions between organisms with high spatial (4K: 3840x2160 pixels) and temporal resolution (up to 120 fps).This technique is particularly useful for gelatinous organisms because of their large (centimeters in length), transparent bodies.Further, the measurements are not subject to experimental artifacts produced in laboratory studies.This method is useful for anyone seeking detailed brightfield images of organisms or nonliving material (e.g. marine snow) in the natural environment.