A search for aperiodic millisecond variability in Cygnus X-1

We have conducted a search for aperiodic millisecond variability in the int egrated 1-25 keV X-ray region of Cyg X-l. We have examined HEAO A-1 archiva l data and Rossi X-Ray Timing Explorer (RXTE) guest observer data for evide nce of excess power above the Poisson noise floor using the relative integr al power analysis and the Fourier transform method. Our results are in disa greement with the results of Meekins and coworkers. We attribute the discre pancy to an instrumental effect for which Meekins and coworkers did not app ly a correction. With the correction we see no evidence for excess power ab ove 25 Hz in the HEAO A-1 data. Our analysis of RXTE data is in agreement w ith previously published results of different data sets and shows no indica tion of excess power above 30 Hz.