The cooperative binding of the Escherichia coli DNA unwinding protein to single-stranded DNA has been studied by electron microscopy. Analysis of the electron microscopic data by means of a simple statistical mechanical model has yielded a value of 3.8-7.6 X 10(10) l./mol for the cooperative binding constant in 0.15 M NaCl. Studied under elevated salt conditions have shown that the average DNA protein complex length is 50% of the length found at 0.04 or 0.15 M NaCl.