['Microtubules (MTs) are protein nanotubes comprised of straight protofilaments (PFs), head to tail assemblies of αβ-tubulin heterodimers. Previously, it was shown that Tau, a microtubule-associated protein (MAP) localized to neuronal axons, regulates the average number of PFs in microtubules with increasing inner radius <R', 'We report a synchrotron SAXS and TEM study of the phase behavior of microtubules as a function of varying concentrations of paclitaxel (1/32≤Λ', 'Tau and paclitaxel have opposing regulatory effects on microtubule bundling architectures and microtubule diameter. Surprisingly and in contrast to previous results at Λ'] Does paclitaxel suppress Tau-mediated microtubule bundling in a concentration-dependent manner?