An array of 24 stations, each with 24 crossed dipoles, has been built at the Haystack Observatory in Westford, Massachusetts. This array has been designed to make a sensitive search for the 327 MHz spectral line of deuterium. Since the deuterium line is expected to be about 50 dB weaker than the 1420 MHz hydrogen line, the amelioration of radio frequency interference (RFI) is a major challenge for the performance of the deuterium array. Locally generated RFI from the array and from nearby sites has been reduced by extensive shielding and in some cases by the removal of consumer electronics, like certain digital answering machines that emit strong signals in the 327.3--327.5 MHz band, which is of prime importance for the search. Since almost all the RFI comes from the horizon, the station array has parasitic directors added to the dipoles to reduce the response at the horizon. An RFI monitor with 12 active Yagi antennas pointed every 30¿ in azimuth provides a way of determining the direction of the RFI and yields information on frequencies and time spans that need to be excised from the array data. We describe the RFI excision techniques and the levels of spectral and continuum RFI measured at the observatory.