  Employing linearized Vlasov-Maxwell system, the Weibel instability embedded
in an ambient magnetic field is discussed for a semi-relativistic bi-Maxwellian
distribution hoping such a scenario occurs in some relativistic environments
e.g., gamma-ray burst sources and relativistic jet sources, supernovae, and
galactic cosmic rays where the perpendicular temperature is much dominated over
the parallel . The dispersion relations are analyzed analytically along with
the graphical representation and the estimates of the growth rate are presented
along with the instability threshold condition in the limiting cases i.e.,
xi<=1 (resonant case) and xi>>1 (non-resonant case). It is observed that the
relativistic effect suppresses the instability and also lowers the threshold
for the instability to set in. The ambient magnetic field contribution to
instability appears only in non-resonant case resulting in reduction of growth
rate. However, the effect of the ambient magnetic field is diminished as we go
from the weak relativistic regime to the highly relativistic one. We also note
that the ambient magnetic field generates real oscillations and the
reletivistic effect reduces these oscillations. Further for field free case
i.e., B0=0, the growth rates for Weibel instabilities are also presented in a
semi-relativistic bi- Maxwellian plasma for both the limiting cases.
