Asymmetry modes (m= 1,kz≠ 0) are diocotron‐like modes in finite‐length plasma columns in Malmberg‐Penning traps. We have investigated the modes with a detailed 3‐d particle‐in‐cell (PIC) drift‐kinetic computer simulation. Although PIC simulations do not employ realistic collisions, the simulations in this case reproduce many of the salient features of the data. Particle transport associated with the damping is seen not to be a direct collisional effect, but rather a feature of orbital dynamics associated with transitions from trapped‐to‐untrapped or untrapped‐to‐trapped state relative to the inversion plane of the asymmetry. In the simulations we observe aB−1dependence of the mode frequencies and aB−0.5dependence of the damping constant for large rigidity. We further observe a steepening of the dependence of the decay constant toB−2as the rigidity of the plasma falls below about 2.0. We have also used the simulations to investigate the modes at small seed amplitudes and observe linear flattening in the mode frequency as the seed amplitude becomes small. In contrast, the decay constant does not flatten for small seed amplitude. © 2003 American Institute of Physics