DNA extracted from phage N3 grown in Haemophilus influenzae strain JC9 was examined by velocity and equilibrium centrifugation, electron microscopy and thermal melting curves. This double stranded DNA, molecular weight 26X10(6) daltons, has continuous single strands at 13X10(6) daltons as shown by alkaline sucrose gradient centrifugation. The duplex form readily aggregates in neutral salt solutions at even moderate ionic strength. Aggregation may be due to N3 DNA having "sticky" ends. CsCl equilibrium centrifugation measurements and thermal melting curves indicate a G+C content of 43 mol %.