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 is presented. Measurements were performed using the full detector 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 were presented. We had also compared the data with a dedicated Monte Carlo sample that reproduces the status of the detector in this period.