  A fluid description for collisionless magnetized plasmas that takes into
account the effect of cyclotron resonance has been developed. Following the
same approach as the Landau fluid closure, the heat flux components associated
with transverse electromagnetic fluctuations are approximated by a linear
combination of lower-order moments in wavenumber space. The closure
successfully reproduces the linear cyclotron resonance for electromagnetic
waves propagating parallel to the ambient magnetic field. In the presence of
finite temperature anisotropy, the model gives approximately correct prediction
for an instability destabilized via the cyclotron resonance. A nonlinear
simulation demonstrates the wave growth consistent with the linear theory
followed by the reduction of initial anisotropy, and finally, the saturation of
the instability. The isotropization may be understood in terms of quasi-linear
theory, which is developed within the framework of the fluid model but very
similar to its fully kinetic counterpart. The result indicates that both linear
and nonlinear collisionless plasma responses are approximately incorporated in
the fluid model.
