A highly efficient and reproducible transformation system for orchardgrass (Dactylis glomerata L. cv. Rapido, 2n=4×=28) was established using microprojectile bombardment of highly regenerative, green tissues derived from mature seeds. These tissues, induced from embryogenic callus, were bombarded with a mixture of three plasmids containing the hygromycin phosphotransferase (hpt), phosphinothricin acetyltransferase (bar) and β-glucuronidase (uidA; gus) genes. From 147 individual explants bombarded, 11 independent hygromycin-resistant lines (7.5%) were obtained after an 8- to 16-week selection period using 30–50 mg/l hygromycin B. Of the 11 independent lines, ten (91%) were regenerable. The presence and integration of the transgene(s) were assessed using PCR and DNA blot hybridization. Coexpression frequency of the three transgenes (hpt/bar/uidA) in T0 plants was 20%, and of two transgenes, either hpt/bar or hpt/uidA, 45–60%. Due to greenhouse conditions optimized for the growth of other species, T1 seed has not been obtained from these plants. While the inability to analyze progeny plants precludes the conclusive demonstration of stable transformation, the results of all molecular and biochemical analyses of T0 plants are consistent with the production of stably transformed plants. Frequent change in ploidy level was observed in transformed T0 orchardgrass plants. Plants from only three of the ten independent lines analyzed had the normal tetraploid number of chromosomes (2n=4×=28), while plants from seven lines (70%) were octaploid (2n=8×=56). The octaploid plants had abnormal morphological features, such as narrower, thicker and more upright leaves.