  I use N-body simulations with two mass species of particles to demonstrate
that disk galaxy simulations are subject to collisional relaxation at a higher
rate than is widely assumed. Relaxation affects the vertical thickness of the
disk most strongly, and drives the velocity ellipsoid to a moderately flattened
shape similar to that observed for disk stars in the solar neighborhood. The
velocity ellipsoid in simulations with small numbers of particles quickly
approaches this shape, but shot noise also dominates the in-plane behavior.
Simulations with higher, but reachable, numbers of particles relax slowly
enough to be considered collisionless, allowing the in-plane dispersions to
rise due to spiral activity without heating the vertical motions. Relaxation
may have affected many previously published simulations of the formation and
evolution of galaxy disks.
