Minimum bias physics with ATLAS

The first stable colliding 450 GeV proton beams were delivered by the LHC in December 2009, providing collisions at 900 GeV centre of mass energy. The selection of minimum bias events for the measurement of charged particle multiplicities in data taken by the ATLAS detector during this initial running will be presented. Measurements were performed using the full tracker and with the solenoidal magnetic field on. The selection is designed to ensure a high quality of the measured tracks and rejection of secondary tracks and beam gas and halo background events. Trigger, vertexing and tracking performance will be presented. We also compare the data with a dedicated Monte Carlo sample that reproduces the status of the detector in this period.