Particle simulations of perpendicular (with respect to the ambient magnetic field) propagation in homogeneous time-varying nongyrotropic magnetoplasmas show that the introduction of gyrophase organization in the (electron or ion) particle populations of otherwise stable media can bring about growth of (electron or ion) electrostatic cyclotron harmonic waves and destabilize the extraordinary mode. The kinetic code simulations demonstrate the occurrence of perpendicular instabilities for various types of nongyrotropic distributions, including those observed for heavy ions in the space shuttle environment, and show that gyrophase diffusion accompanies the evolution toward saturation. ¿ 1998 American Geophysical Union