In Paracoccidioides brasiliensis, a dimorphic fungus pathogenic for humans, no significant differences were observed in the phospholipid species of both morphological phases. The species observed were phosphatidylcholine (PC, 30-40 O h ) , p hosp hat idy let hanolami ne (PE, 27-28 O h ) , phosp hat id y lser i ne (16-19%), phosphatidylinositol (1347%) and sphingomyelin (3-50' 0). The main fatty acids found in the yeast (Y) phase were palmitate (56%), linoleate (18%) and oleate (15%), while linoleate predominated (61 YO) in the mycelial (M) phase, followed by palmitate (27 YO) and oleate (7 YO). In the Y phase the main free sterol was ergosta=5,22=dien=3/%ol (82 YO) plus some lanosterol (12 YO) and ergosterol (6%), while in the M phase, the latter predominated (88%), followed by low levels of ergosta-5,22-dien-3/%ol (12 YO). Ajoene [(€,2)-4,5,9-trithiadodeca-l,6,11-triene 9-oxide], a platelet aggregation inhibitor derived from garlic, induced alterations in phospholipid and fatty acid proportions such that PC was reduced to about 18% in both phases and PE increased to 38% (Y phase) or 44% (M phase), suggesting inhibition of PC synthesis. Ajoene also reduced saturated fatty acids (16:O and 18:O) from 67 to 35% in the Y phase, with a corresponding increase in the unsaturated components. This effect was not seen in the M phase.