It is well-known that polyamines, such as spermidine, have the effect of condensing DNA molecules into a toroidal morphology with an outer diameter of 80-100 nm. We have investigated the collapse of long duplex T4 DNA induced by spermidine with use of fluorescence and electron microscopy. We found that giant toroids with an outer diameter of similar to 200 nm are generated from single T4 DNA under a high concentration of spermidine in the buffer solutions with rather high salt.