Simulation results of a Mach 0,5 rectangular jet with a nozzle aspect ratio of five and a Reynolds number of 2000 based on the narrow nozzle width are presented. The focus is on the flow development in the near-nozzle region and on the flow-induced acoustic radiation from the jet. Perturbations based on most unstable modes of the laminar inflow pro_le predicted by linear stability theory are enforced at the inflowto initiate transition. A high-order simulation code was developed and validated and direct numerical simulations (DNS) as well as large-eddy simulations (LES) were performed. For the LES the approximate deconvolution model (ADM) is applied, which is based on an approximate inversion of the LES _lter. DNS results show rapid development of the rectangular jet in the transitional regime towards an axisymmetric shape. The transition process of the jet is well reproduced by the LES and mean flow data show very good agreement with the altered DNS data. Acoustic analysis of DNS data based on Lighthill's analogy confirms typical azimuthal-directive behavior of non-axisymmetric jets along the minor axis of the jet in the main radiation direction. Acoustic far-field prediction based on LES data reproduces the dominant lower frequencies up to a cutoff frequency.