We use the zCOSMOS galaxy overdensity field to study the biasing of galaxies in the COSMOS field. By comparing the probability distribution function of the galaxy density contrast δ g to the lognormal approximation of the mass density contrast δ, we obtain the mean biasing function b(δ, z, R) between the galaxy and matter overdensity fields and its second moment b an b. Over the redshift interval 0.4 < z < 1 the conditional mean function δ g |δ = b(δ, z, R)δ is of a characteristic shape, requiring nonlinear biasing in the most overdense and underdense regions. Taking into account the uncertainties due to cosmic variance, we do not detect any significant evolution in the δ g |δ function, but we do detect a significant redshift evolution in the linear biasing paramete b from 1.23 ± 0.11 at z ∼ 0.55 to 1.62 ± 0.14 at z ∼ 0.75, for a luminosity-complete sample of M B < −20 − z galaxies. Thê b parameter does not change significantly with smoothing scale between 8 and 12 h −1 Mpc, but increases systematically with luminosity (at 2σ –3σ significance between the M B < −20.5 − z and M B < −20 − z samples). The nonlinearity paramete b b is offset from unity by at most 2%, with an uncertainty of the same order. Th b b parameter does not show any significant redshift evolution, dependence on the smoothing scale or on the luminosity. By matching the linear bias of galaxies to the halo bias, we infer that the M B < −20 − z galaxies reside in dark matter halos with a characteristic mass of about (2.6 − 5.6) × 10 12 M with a small dependence on the adopted bias–mass relation. Our detailed error analysis and comparison with previous studies lead us to conclude that cosmic variance is the main contributor to the differences in the linear bias measured from different surveys. While our results support the general picture of biased galaxy formation up to z ∼ 1, the fine-tuning of the galaxy formation models is still limited by the restrictions of the current spectroscopic surveys at these redshifts.