Microsegregation in individual single chains of T4 DNA in solutions of poly(2-vinylpyrrolidone) (PVP) was observed by fluorescent microscopy. After mixing solutions of DNA with PVP, single DNA chains exhibited intrachain segregation with multiple mini-globules connected by narrow flexible chains, The microsegregated form of DNA was observed only in solutions of high-molecular-weight PVP (M(w) similar to 220 000), while in solutions of short chain PVP (M(w) similar to 10 000), only the randomly coiled conformation was found. The average lifetime of microsegregated DNA chains was more than several days while some of the microsegregated DNA molecules changed into a completely collapsed state.