  We use the overdensity field reconstructed in the volume of the COSMOS area
to study the nonlinear biasing of the zCOSMOS galaxies. The galaxy overdensity
field is reconstructed using the current sample of ~8500 accurate zCOSMOS
redshifts at I(AB)<22.5 out to z~1 on scales R from 8 to 12 Mpc/h. By comparing
the probability distribution function (PDF) of galaxy density contrast delta_g
to the lognormal approximation of the PDF of the mass density contrast delta,
we obtain the mean biasing function b(delta,z,R) between the galaxy and matter
overdensity field and its second moments b(hat) and b(tilde) up to z~1. Over
the redshift interval 0.4<z<1 the conditional mean function <delta_g|delta> =
b(delta,z,R) delta is of the following characteristic shape. The function
vanishes in the most underdense regions and then sharply rises in a nonlinear
way towards the mean densities. <delta_g|delta> is almost a linear tracer of
the matter in the overdense regions, up to the most overdense regions in which
it is nonlinear again and the local effective slope of <delta_g|delta> vs.
delta is smaller than unity. The <delta_g|delta> function is evolving only
slightly over the redshift interval 0.4<z<1. The linear biasing parameter
increases from b(hat)=1.24+/-0.11 at z=0.4 to b(hat)=1.64+/-0.15 at z=1 for the
M_B<-20-z sample of galaxies. b(hat) does not show any dependence on the
smoothing scale from 8 to 12 Mpc/h, but increases with luminosity. The measured
nonlinearity parameter b(tilde)/b(hat) is of the order of a few percent (but it
can be consistent with 0) and it does not change with redshift, the smoothing
scale or 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 haloes with a
characteristic mass of about 3-6 x 10^12 Msol, depending on the halo bias fit.
