Small broomrape, a holoparasitic weed, is a relatively new weed introduction in the Pacific Northwest that has contaminated a limited number of red clover fields in Oregon. Greenhouse and field studies were conducted to evaluate small broomrape response to common crop and weed species in the Pacific Northwest. Host species in the greenhouse or field study included alfalfa, arrowleaf clover, carrot, celery, common vetch, crimson clover, lettuce, prickly lettuce, red clover, spotted catsear, subterranean clover, white clover, and wild carrot. False-host species included barley, birdsfoot trefoil, creeping bentgrass, cucumber, field corn, fine fescue, flax, Italian ryegrass, nasturtium, oat, orchardgrass, perennial ryegrass, snap bean, sugar pea, sunflower, sweet corn, tall fescue, tomato, and wheat. Non-host species included sugar beet and curly dock. The greenhouse polyethylene bag system provided a rapid and inexpensive screening for plant species host status to small broomrape. Germination and attachment to host roots are initiated by chemical exudates, that may change concentration in response to nutrient availability and microorganisms. Red clover was grown in varying concentrations of ammonium sulfate fertilizer with and without Rhizobium inoculation, and with small broomrape seeds. Neither Rhizobium inoculation nor ammonium concentration influenced the number of small broomrape attachments to red clover roots. A survey was conducted of red clover seed growers with small broomrape-contaminated fields in the Pacific Northwest. Red clover seed from six respondents were cleaned at the same cleaning facility, and the same respondents purchased their seed stock from this cleaning facility. Small broomrape was not identified in red clover fields prior to or during the first clover seed harvest of fall planted red clover in small broomrape-contaminated sites. / Graduation date: 2003