We have used microprojectile bombardment to obtain transient expression of the reporter gene -glucuronidase in differentiating wood (secondary xylem) of loblolly pine (Pinustaeda L.), thereby providing a method for studying expression of introduced DNA in this important tissue. -Glucuronidase activity can be observed in different cell types, including tracheids, ray parenchyma, and axial parenchyma associated with resin canals. Microprojectile bombardment can be used to identify active promoters and to compare the relative activities of different promoters in specific cell types. We have studied the expression of three promoter gene fusions by bombardment of loblolly pine stem sections. Wood samples bombarded with an abscisic acid inducible (Em) promoter-glucuronidase fusion contained nearly 10 times the number of stained cells as those bombarded with two more commonly used promoter fusions (nopaline synthase or cauliflower mosaic virus 35S). Microprojectile bombardment of differentiating wood should be useful for studying promoters and gene constructs in future attempts to modify wood properties by genetic engineering.