Initial interactions of Striga asiatica with a susceptible host and non-host plants were examined by histological methods. Haustorial development was initiated when radicles of S. asiatica were placed in contact with host or nonhost roots. Reorganization of the S. asiatica root apical meristem was rapid and involved the formation of a distal group of cells that penetrated the host or nonhost root. Penetration of the epidermis of the host (sorghum) roots and advance into the cortex occurred within 24 to 48 h of inoculation. Penetration of the endodermis by the developing endophyte was delayed for 72 to 96 h after initial contact. However, upon penetration vascular continuity was established between parasite and host. In contrast, interactions with nonhosts provided evidence of active resistance mechanisms. Penetration of lettuce, marigold, and cowpea roots by S. asiatica was most frequently arrested in the cortex, and endophytic cells were necrotic 72 h after inoculation. Some species-specific differences were observed in the reactions of nonhosts to penetration, although in their general nature the interactions with S. asiatica were similar.