The lobula giant motion detector (LGMD) in the locust visual system is a wide-field, motion-sensitive neuron that responds vigorously to objects approaching the animal on a collision course. We investigated the computation performed by LGMD when it responds to approaching objects by recording the activity of its postsynaptic target, the descending contralateral motion detector (DCMD). In each animal, peak DCMD activity occurred a fixed delay delta (15 </= delta </= 35 msec) after the approaching object had reached a specific angular threshold thetathres on the retina (15 degrees </= thetathres </= 40 degrees). thetathres was independent of the size or velocity of the approaching object. This angular threshold computation was quite accurate: the error of LGMD and DCMD in estimating thetathres (3.1-11.9 degrees) corresponds to the angular separation between two and six ommatidia at each edge of the expanding object on the locust retina. It was also resistant to large amplitude changes in background luminosity, contrast, and body temperature. Using several experimentally derived assumptions, the firing rate of LGMD and DCMD could be shown to depend on the product psi(t - delta). e-alphatheta(t-delta), where theta(t) is the angular size subtended by the object during approach, psi(t) is the angular edge velocity of the object and the constant, and alpha is related to the angular threshold size [alpha = 1/tan(thetathres/2)]. Because LGMD appears to receive distinct input projections, respectively motion- and size-sensitive, this result suggests that a multiplication operation is implemented by LGMD. Thus, LGMD might be an ideal model to investigate the biophysical implementation of a multiplication operation by single neurons.