  The multi-planetary system HD128311 hosts at least two planets. Its dynamical
formation history has been studied extensively in the literature. We reanalyse
the latest radial velocity data for this system with the affine-invariant
Markov chain Monte Carlo sampler EMCEE. Using the high order integrator IAS15,
we perform a fully dynamical fit, allowing the planets to interact during the
sampling process. A stability analysis using the MEGNO indicator reveals that
the system is located in a stable island of the parameter space. In contrast to
a previous study, we find that the system is locked in a 2:1 mean motion
resonance. The resonant angle $\varphi_1$ is librating with a libration
amplitude of approximately 37{\deg}. The existence of mean motion resonances
has important implication for planet formation theories. Our results confirm
predictions of models involving planet migration and stochastic forces.
