  We present the first in a series of papers describing the X-ray properties of
a sample of 18 Seyfert 1 galaxies, using data obtained by \asca. The imaging
data 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 but
one of the Seyferts show evidence for variability on timescales of
minutes-hours, with the amplitude anti-correlated with the source luminosity,
confirming previous results. In at least 8 sources, there is evidence that the
variability 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 soft
band. Ultra-rapid variability, implying significant power at frequencies $>
10^{-3}$~Hz is detected in at least 5 sources, but is difficult to detect in
most cases, due to the sampling and signal-to-noise ratio. In Mrk 766 and
MCG-6-30-15 there is also an indication that the high--frequency power--spectra
are variable in shape and/or intensity. There is similar evidence in NGC 4151,
but on longer time scales.
