Many aspects of individual and collective cell motion are still poorly understood and physicalmodels dealing with both aspects appear clearly as valuable tools for understanding cellbiomechanical behavior. We theoretically analyze and simulate individual cell mechanicalproperties and the related chemotactic behavior of single cells collectively involved in anaggregation process. Cell objects are defined as three-dimensional elastic bodies moving on aviscous medium and submitted both to internal cohesive forces and to external attractiveforces (gravity and chemoattraction). We first investigate individual cell mechanical responseto externally applied forces and compare the simulated cell object deformations toexperimental data obtained with optical tweezers. We then examine the simulated cellpopulation reorganization submitted to a chemoattraction field over a 2D solid planesimulating a glass coverslip. Simulations are carried out for different values of cellchemotactic response. We especially simulate cell sorting between pre-spore and pre-stalkcells during Dictyostelium discoideum aggregation process by considering two cellpopulations exhibiting differential cell-cell interactions. We conclude that our physical-objectoriented (POO) approach for modelling individual cell cytomechanics also satisfactorilyreproduces the two-dimensional aggregation at a cell population level. Because individual cellmechanical behavior can be compared to a wide range of cell micro-rheology experiments,the cell object parameters can be estimated. This framework should thus be adequate forbiologically realistic multi-scale analyses leading to a better understanding of how, throughmodulation of mechanical factors, individual cell behavior controls collective cellreorganization.