  We present the results of a deep search for pulsations from the nearby
isolated neutron star RX J1856.5-3754 using the 450 ks Director's Discretionary
Time Chandra observation completed on 2001 Oct 15. No pulsations were detected.
We find a 99% confidence upper limit on the pulsed fraction of ~4.5% for
worst-case sinusoidal pulsations with frequency <~50 Hz and frequency
derivatives -5x10^-10 <= f-dot <= 0 Hz/s. The non-detection of pulsations is
most likely due to an unfavorable viewing angle or emitting geometry. Such
emitting geometries are much more likely to occur for more ``compact'' neutron
stars which show increased gravitational light-bending effects. In this case,
the non-detection implies a radius/mass ratio for RX J1856.5-3754 of R/M <~10
km/Msun.
