  The Weibel instability could be responsible for the generation of magnetic
fields in various objects such as gamma-ray bursts, jets from active galactic
nuclei, and clusters of galaxies. Using numerical simulations, the development
of the Weibel instability at a temperature gradient is studied. It is found
that current sheets are first generated at the gradient, and then they are
rounded off and turn into current filaments. During this process, return
currents are generated around the filaments and they prevent filaments from
merger. The magnetic fields around the filaments persist at least until
t~8000/\omega_p, where \omega_p is the plasma frequency, and it is very likely
that they survive for a much longer time.
