Using air-dried preparations of the testis and ovary, karyotypes were analyzed and compared to each other in two species of filarial parasites, Brugia pahangi and B. malayi. Both species had a diploid number of 10 chromosomes and were karyotypically very similar. C-banding analyses disclosed that the sex-determining mechanism of these species was of the XY-XX type, where the X chromosome was the largest, and the Y chromosome was of medium-size.