Low, medium and high densities of western flower thrips,Frankliniella occidentalis (Pergande), were established inthree greenhouses at the Greenhouse and Processing Crops ResearchCentre, Ontario, Canada, in 1996 and 1998 to develop economic injurylevels for thrips on greenhouse cucumber. Thrips densities weremonitored weekly using yellow sticky traps and flower counts. Fruit washarvested twice a week, graded for size, weighed, and rated for thripsdamage using three damage categories. Significant yield reduction wasdetected 4 wk after severe fruit damage was observed in the high andmedium thrips density treatments in 1996 and 7 wk in 1998. Percentageof severe damaged fruit (P_F3) has significant linearrelationships with the adult thrips density (x) that was sampled bysticky traps 1 wk before harvest (P_F3 = −0.2533 0.0828x) and that was sampled by flower counts 2 wk before harvest(P_F3 = − 0.2025 0.5490x). Based on theregression equations, economic injury levels, expressed as adult thripsper sticky trap per day or adult thrips per flower, were calculated forvarious combinations of control costs, yield potential and fruitprices. The economic injury levels for F. occidentalisranged from 20 to 50 adults per sticky trap per day or 3 to 7.5 perflower as determined under average greenhouse production conditions inOntario, Canada.