PHOTOGRAPHY OF AQUATIC organisms poses special problems-and rewards-to the biologist. Details of their structure, behavior and environment can be documented clearly. Yet water is a demanding medium to penetrate and to retain in place, and the interface between water and air refracts and reflects light, causing special problems to the photographer. The small size of many aquatic organisms calls for the use of modern techniques of photomicrography and macrophotography. In contrast to photography of organisms on dry land, or in the air, photography of aquatic organisms in their element often requires special equipment. This equipment can range from modest, homemade aquaria to elaborate watertight housings for cameras, flash and lightmeter, remote controlled underwater television cameras, and even specially equipped submarines. Don't get scared off. Much can be achieved in photographing aquatic organisms without having to resort to million dollar budgets. After all, there isn't room in the world for too many Jacques Cousteaus. It is practically impossible to give a full visual record of any organism in just one picture. So, when considering the photography of an organism it is good to keep three H's in mind: the Habitat, the Habitus, and the Habits. Many photographers try to include an impression of the habitat with a careful portrait of the organism (habitus). The results are often charming and interesting, but as often the picture is somewhat confused because of the dominance of the background over the main object. Although inclusion of the organism in a visualization of the habitat is fine when possible, it is important to present the organism clearly in its own right. When it comes to the portrait, I personally prefer the surroundings as unobtrusive as possible. Beyond this, and to demonstrate the habits (general movements, feeding, courting, mating and so forth) a series of photographs is usually needed, sometimes with long intervals between the exposures. The choice of photographic technique depends critically on the dimensions of the organisms, and also varies depending on whether they are static or dynamic, and the light conditions (is the ambient light satisfactory or should it be supplemented or replaced with floodlamps or flash?) The term static refers to any non-moving organisms, dynamic, to the organism in movement, whether the motion is made by the organism itself or is caused by wind or current. In the following discussion, it is assumed that we are dealing only with dynamic, living organisms and that they are freely moving (often under controlled conditions). Means for anesthetizing invertebrates, fish, and other aquatic organisms exist (Behringer 1973; Morholt, Brandwein and Joseph 1966). In order to bring the photographic record as close as possible to real life, I try to avoid using such methods.