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We present the results from the RXTE observations of Cygnus X-1 in its highstate. In the energy range of 2-200 keV, the observed X-ray spectrum can bedescribed by a model consisting of a soft blackbody component and a brokenpower-law with a high energy cutoff. The low energy spectrum (below about 11keV) varies significantly from observation to observation while the high energyportion changes little. The X-ray flux varies on all timescales down tomilliseconds. The power density spectrum (PDS) can be characterized by excessred noise (``1/f'') at low frequencies and a white noise component that extendsto 1-3 Hz before being cut off. At higher frequencies, the PDS becomespower-law again, with a slope of roughly -2 (i.e., ``1/f^2''). Broad peaks inthe range of 3-9 Hz are present, and might be due to quasi-periodicoscillations. The PDS shows interesting spectral dependence: the 1/f componentbecomes more prominent when the low-energy spectrum becomes softer. Thedifference in the observed spectral and timing properties between the low andhigh states is qualitatively consistent with a simple ``fluctuating corona''model.Comment: 11 pages including 3 figures, LaTeX (aaspp4.sty). Accepted for  publication in ApJ Letter

[ITAL]Rossi X-Ray Timing Explorer[/ITAL] Observation of Cygnus X-1 in Its High State