To examine the extent of genetic variation in Biomphalaria glabrata, the intermediate host of Schistosoma mansoni, specimens from seven Brazilian isolates were studied. Regions of genomic DNA were amplified using short, arbitrarily selected oligonucleotide primers under low-stringency conditions (random amplified polymorphic DNAs--RAPDs) and analyzed by polyacrylamide gel electrophoresis followed by silver staining. The RAPD profiles of snails of the same isolate were relatively homogeneous, with the majority of bands being common to all individuals studied. In contrast, the profiles of snails from different isolates were quite distinct, with less than 10% of amplified DNA fragments being common to all of the specimens studied. This was found to be true irrespective of the primer, indicating that B. glabrata is remarkably genetically heterogeneous. The data are consistent with the view that genetic drift plays a major role in the genetic structure of populations of B. glabrata due to their ability to reconstitute populations from very small numbers of individuals by self-fertilization. The great variety of polymorphic genetic markers identified in this restricted survey indicates that RAPD analysis may make a major contribution to the study of the genetics of Biomphalaria.2099-12-31