Using fluorescence microscopy, we studied the folding transition induced by spermidine (3+) in individual giant DNAs isolated from pig liver, with an average size of 60 kbp varying between 15 and 150 kbp. We found that individual DNA chains undergo a large discrete transition, or switching, from an elongated coil state to a folded compact state. Interestingly, the width Of the region of coexistence is rather narrow with respect to the spermidine concentration, despite the polydispersity of the DNA specimen. The weak dependence of the manner of the folding transition on molecular weight is discussed based on a simple mean-field theory. (C) 2002 Elsevier Science B.V. All rights reserved.