Annual grass weeds are increasing as a dominant weed species in the western United States, Canada and Mexico. Downy brome, one of the most widespread, invasive annual grass weeds, negatively affects cereal yields, reduces forage quality in grazing lands, degrades rangelands, and increases the fire frequency of western lands. This chapter reviews how naturally-occurring bacteria were screened to find those that suppress downy brome but do not harm native plants and crops, and how one such bacterial strain, Pseudomonas fluorescens strain ACK55, was identified as able to a reduce downy brome root formation, root growth, and tiller initiation. It then discusses long-term field trials in the western US, in which application of the bacteria resulted in almost complete suppression of downy brome for three to five years after one application, when desirable plants were present.