To understand the polymerization dynamics of FtsZ, a bacterial cell division protein similar to tubulin, insight is required into the nature of the nucleotide bound to the polymerized protein. In a previous study, we showed that the FtsZ polymers contain mostly GDP. A recent study challenged this result, suggesting that the polymerized FtsZ is in a GTP-bound state. Here, we show that, when radiolabelled [γ-32P]-GTP is used to polymerize FtsZ, GTP is hydrolysed instantaneously. The FtsZ polymer contains both GDP and the radiolabelled inorganic phosphate.