Ion temperature gradient drift instabilities have been investigated using gyrokinetic particle simulation techniques for the purpose of identifying the mechanisms responsible for their nonlinear saturation as well as the associated anomalous transport. For simplicity, the simulation has been carried out in a shear‐free slab geometry, where the background pressure gradient is held fixed in time to represent quasistatic profiles typical of tokamak discharges. It is found that the nonlinearly generated zero‐frequency responses for the ion parallel momentum and pressure are the dominant mechanisms giving rise to saturation. This is supported by the excellent agreement between the simulation results and those obtained from mode‐coupling calculations, which give the saturation amplitude as ‖e&Fgr;/Te‖ &bartil;(‖&ohgr;l+i&ggr;l‖/&OHgr;i)/(k⊥&rgr;s)2, and the quasilinear thermal diffusivity as &khgr;i&bartil;&ggr;l/k2⊥, where &ohgr;land &ggr;lare the linear frequency and growth rate, respectively, for the most unstable mode of the system. In the simulation, the time evolution of &khgr;iafter saturation is characterized by its slow relaxation to a much lower level of thermal conduction. On the other hand, a small amount of electron–ion collisions, which has a negligible effect on the linear stability, can cause significant enhancement of &khgr;iin the steady state.