The path chosen by a toad (Bufo viridis) was examined when a barrier was placed between it and its food. The toad plans its route before it sets out, and so its route reveals something of its perceptions of its immediate environment. If there is a gap in the barrier wider than 3 cm (the width of the toad's head), the toad aims for the gap, if not, it detours round the end of the barrier. Provided that the toad starts no further than 20 cm from the fence, its choice between detour and direct approach is governed by the physical size of the gap and not by the angle that the gap subtends on the retina, indicating that the toad has size constancy. Similarly, it is less likely to detour round wide barriers than narrow ones and again its behaviour is controlled by the real width of the barrier and not by the retinal subtense.