In order to elucidate whether relatively primitive primates show a similar topological organization of substrates mediating defense and flight within the ventromedial hypothalamus as has been found in other mammals, 15 marmosets (Callithrix jacchus) were chronically implanted with a total of 57 electrodes for electrical stimulation. Experiments took place in a familiar environment and under systematically altered conditions. All anatomical sites yielding vocal threat and short attacks were found to be situated in the n. ventromedialis. Flight was elicited from a region immediately lateral to this nucleus and from sites in the posterior hypothalamus. Thus, it appears that the topological organization of defense and flight behavior in cat, oppossum and marmoset are essentially similar.