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We present the first in a series of papers describing the X-ray properties ofa sample of 18 Seyfert 1 galaxies, using data obtained by \asca. The imagingdata reveal a number of serendipitous hard X-ray sources in some source fields,but none contribute significantly to the hard X-ray flux of the AGN. All butone of the Seyferts show evidence for variability on timescales ofminutes-hours, with the amplitude anti-correlated with the source luminosity,confirming previous results. In at least 8 sources, there is evidence that thevariability amplitude below 2 keV is greater than that in the hard X-ray band,perhaps indicating variable components other than the power-law in the softband. Ultra-rapid variability, implying significant power at frequencies $>10^{-3}$~Hz is detected in at least 5 sources, but is difficult to detect inmost cases, due to the sampling and signal-to-noise ratio. In Mrk 766 andMCG-6-30-15 there is also an indication that the high--frequency power--spectraare variable in shape and/or intensity. There is similar evidence in NGC 4151,but on longer time scales.

ASCAObservations of Type 2 Seyfert Galaxies. I. Data Analysis Results