The development of a polymerase chain reaction (PCR) based fingerprinting method for the characterization of Giardia duodenalis isolates is described. The method uses three different PCRs; one is specific for the A ("Polish") major group of G. duodenalis isolates, another is specific for the B ("Belgian") group of isolates; and one amplifies a fragment of the glutamate dehydrogenase gene present in all G. duodenalis isolates. The PCRs perform highly sensitively on DNA from cultured trophozoites. Isolates were further characterized by restriction-fragment-length polymorphism (RFLP) analysis of the PCR products. In this way, representative isolates from the A and B groups could be grouped together into a number of subgroups. The stability of the genotypes with time and the reproducibility of the two methods were tested on cloned and subcloned lines from a number of isolates and proved to be highly satisfactory. The PCR/RFLP method was evaluated on cysts derived from a number of human patients. It is concluded that the PCR fingerprinting method described in this paper provides a reliable characterization method for Giardia isolates and has the potential to be used as a direct method of typing G. duodenalis cysts from feces.