Host plant use by nymphs and adults of the nonnative species Halyomorpha halys (Stål) (Hemiptera: Pentatomidae) was investigated proximal to the location of its introduction, Allentown, PA. The seasonality of H. halys in the United States had not been thoroughly studied before this work. It is reported to have ≈300 host plants in its native range that could make control and identification of small populations difficult. Weekly beat samples were conducted beginning at petal fall (mid-April) in Pyrus spp. until the first frost (mid-October) from 2005 to 2007 on a variety of ornamental trees, shrubs, and agricultural crops. Egg masses were first observed on Paulownia tomentosa Thunb. the first week of June. In 2006 and 2007, Fraxinus americana L. was an important mid- and late season host for adults. Nymphal abundance differed seasonally. P. tomentosa supported high densities during the early season, whereas Viburnum opulus variety americanum Aiton was the preferred mid-season host, and Viburnum prunifolium L. and Rosa rugosa Thunb. had the highest densities of nymphs during the late season. Abundance of nymphs was strongly associated with maturing fruit or pods. All plants surveyed supported populations of H. halys, suggesting a large host range. In late August, a large adult population peak was observed (850—1,000 degree days [DD]), shortly after the DD accumulation for development to imaginal eclosion, supporting hypotheses that H. halys is likely univoltine in this region. Relative to native pentatomid species, H. halys was by far the predominant species collected in samples on ornamental and cultivated crops.