Orchinol, loroglossol, and several related phenanthrenes and stilbenes obtained by laboratory synthesis were tested for antifungal activity, primarily in spore germination assays against Monilinia fructicola and Phytophthora infestons. The most active stilbene tested was 3,5-dimethoxy-3′-hydroxystilbene (mean effective dose (ED50) 5 × 10−5 M), at least comparable with the reported activity of the stilbene phytoalexin, pinosylvin. All the phenanthrenes and dihydrophenanthrenes tested were active, although assays were complicated by solubility problems. ED50 for orchinol was (5 × 10−5 M) and loroglossol had similar activity against P. infestons. Several phenanthrenes had higher activity, notably dehydroorchinol (ED50 vs. P. infestons 5 × 10−6 M), and warrant further study. A striking feature of the response of the test fungi to the phenanthrenes and dihydrophenanthrenes was the production of distorted and ruptured germ tubes, which may indicate a direct interference with growth processes at the tip. No evidence was obtained for translocation of orchinol in higher plant tissue.