Two male and two female Brugia malayi were collected from Mastomys natalensis, and cross sections from various regions of these worms were studied. The body wall of the female worm is thin throughout its length, whereas in the midbody and posterior region of the male worm the somatic muscles form a thick layer. In both sexes the intestine is thickest immediately behind the oesophagus and tapers posteriorly. The lumen was empty in all sections. In both genital tubes of the female worm a successive row of developmental stages could be observed. The rather short ovaries lie in the posterior region. The fertilisation chambers and the following regions are the thickest portions of the uteri. The testis is situated in the anterior region of the male worm, whereas in the midbody and posterior regions the vas deferens contains stretched spermatozoa. Compared to other filariae the sexual dimorphism is not very pronounced in this species.