A flat, relatively thin (9 mm) xenon-filled multiwire proportional counter with two-dimensional, 2/~s delay line readout of a 270 x 300 mm active area has been developed for use as a position-sensitive area X-ray detector in the 8 keV energy region (Cu Ka) used in crystallographic structure work with large biological molecules. Its quantum detection efficiency for 8 keV Xray photons is about 0.5, a value which is spatially uniform to within _+ 2%. Its dead-time loss fraction at a typical data collection rate of 30 000 photons s-1 is 12%. The detector has spatial resolution for X-rays of 0.6 mm FWHM in the horizontal direction and 2 ram, the anode wire spacing, in the vertical direction. The effects of parallax are found to be limited and do not seriously increase the apparent size of the diffracted beams. The position sensitivity of this detector is geometrically linear to within 0-5 mm across its active surface. Routine maintenance of the detector requires the attention of a skilled technician but is not time consuming. For four years, this detector has been used to measure millions of reflection intensities from crystals of many different proteins. The down time due to the detector has averaged less than four days per year, considerably less than the down time of other components of the data collection system. Four new protein structures have now been solved using data from this detector. Also, a considerable amount of data have been collected at higher resolution or at different temperatures with crystals of other proteins.

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