In the past few years several attempts have been made to obtain sunflower genetic transformation using different tissues such as meristems, shoot axes, hypocotyls, cotyledons of young seedlings and immature embryos. Transgenic plants were obtained in some instances but the efficiency and reproducibility were low. The common problem was the difficulty to establish an efficient regeneration system strictly correlated with the transformation system. Several reports indicate sunflower cotyledons as one of the most effective explants for plant regeneration but unlike other sunflower tissues this material is not easily transformed by Agrobacterium. Particle bombardment is an important tool for monocot transformation and could be an interesting alternative also for sunflower cotyledons. Particle delivery system currently sold commercially has a limited focusing ability whereas cotyledons exhibited a confined regenerable area restricted to the embrionic axes. To overcome this problem a stainless steel focusing device that can be applied to the DuPont/BioRad PDS1000/He apparatus was constructed. This device was tested in a series of ß-glucuronidase expression carried out on sunflower cotyledons, cv. HA89 and experimental lines. Endogenous GUS-like activity was abolished by changing the sample buffer and by performing the reaction at 56℃ after having pre-equilibrated explants and buffer for 90 min. The device enabled to confine and concentrate more than 90% of the transformation events in a ring-shaped area at 5-6 mm from the target center. Without the device transformation was very low and scattered over a much wider surface. On the basis of these data an appropriate arrangement of the cotyledons could improve dramatically the probability to transform competent cell for regeneration; this could be even more evident if sunflower genotypes with a high frequency of shoot regeneration are used.