Target localization and verification of the treatment position is important for the accurate delivery of conformal radiotherapy. The bladder in particular is a deformable structure whose shape and position continually varies throughout a course of radiation treatment as a result of bladder filling. We report a novel technique of organ localization using gold seeds as fiducial markers that are implanted into the bladder using a specially adapted applicator that is passed through a rigid cystoscope. The seeds are readily apparent on electronic portal imaging taken at the time of radiotherapy and can thus act as a surrogate for bladder position. The feasibility and technical aspects of performing such a procedure on eight patients were assessed. In all of the patients, some of the seeds were visible on the planning CT scan and remained within the bladder wall throughout the course of radiotherapy treatment. The drop-out rate was minimized by the use of cystodiathermy at the site of seed insertion. It was possible to place the seeds in both areas of normal and diseased bladder tissue. The procedure was associated with minimal toxicity. This technique will form the basis for planning further studies on bladder localization.