A new technique using a single camera and shadows to determine 3-D spatial positions of fishes in the laboratory is described. The apparatus consisted of a large aquarium (2·0 × 1·5 × 0·4 m), a wide-angle camera mounted above and two light sources to cast shadows to either side of the fish. Using image analysis and vector mathematics, aquarium objects were plotted within 1·5 cm of their actual location along the x-, y- and z-axis. The technique was also successful in quantifying changes in 3-D spatial pattern of juvenile fish, Atlantic cod Gadus morhua(7·4–8·6 cm standard length, LS) and cohabitant piscivorous shorthorned sculpin Myoxocephalus scorpinus(12·0–25·8 cm LS), at these same viewing fields. The new technique should have a wide application, largely because it is potentially less expensive, laborious and invasive than alternative methods for determining 3-D positions of fishes.