The accurate tracking of an animal's movements and postures through time has broad applicability to questions in neuroethology and animal behavior. In this paper we describe methods for precision body modeling and model-based tracking of non-rigid animal movements without the use of external markers. We describe the process of obtaining high-fidelity urethane casts of a model organism, the weakly electric knifefish Apteronotus albifrons, and the use of a stylus-type 3-D digitizer to create a polygonal model of the animal from the cast. We describe the principles behind markerless model-based tracking software that allows the user to translate, rotate, and deform the polygon model to fit it to digitized video images of the animal. As an illustration of these methods, we discuss how we have used model-based tracking in the study of prey capture in nocturnal weakly electric fish to estimate sensory input during behavior. These methods may be useful for bridging between the analytical approaches of quantitative neurobiology and the synthetic approaches of integrative computer simulations and the building of biomimetic robots.