Microscopic studies on single-spore-infected sunflower seedlings were carried out in order to study the homothallic or heterothallic nature of sexual reproduction in sunflower downy mildew, Plasmopara halstedit. The formation of oospores was found in the host tissue of all plants infected with single zoospores of three different pathotypes and originating from different geographic areas. This documents the homothallic nature of sunflower downy mildew and is of epidemiological importance. since a single infection in a field can lead to the contamination of the soil with oospores. Initiation of sexual reproduction started two to three weeks after inoculation of two-day-old seedlings with single, micromechanically selected zoospores. Oogonia and antheridia developed in close proximity to each other at the same hypha. The cytoplasm of the gametangia was separated from that of the adjacent hypha through the formation of a septum which consists of callose-like material according to histochemical staining with resorcin.