  Observations with the Rossi X-ray Timing Explorer and the Jodrell Bank,
Parkes, and Green Bank telescopes have enabled us to determine the time delay
between radio and X-ray pulses in the two isolated pulsars B1821-24 and
B1509-58. For the former we find that the narrow X-ray and radio pulse
components are close to being coincident in time, with the radio peak leading
by 0.02 period (60 +/- 20 microsec), while the wide X-ray pulse component lags
the last of the two wider radio components by about 0.08 period. For the latter
pulsar we find, using the standard value for the dispersion measure, that the
X-ray pulse lags the radio by about 0.27 period, with no evidence for any
energy-dependence in the range 2-100 keV. However, uncertainties in the history
of the dispersion measure for this pulsar make a comparison with previous
results difficult. It is clear that there are no perceptable variations in
either the lag or the dispersion measure at time scales of a year or less.
