Microbial organisms often produce biologically active and chemically intriguing secondary metabolites. To isolate and characterize compounds of novel activity and structure, bioassay- guided fractionations were employed on a variety of natural products sources. Active compounds were characterized by spectroscopic analysis and X-ray diffraction methods. As a result of these studies, six projects were completed, resulting in the purification of a diverse range of compounds containing activity in a variety of assays. A coumarin and parahydroxyphenylethanol were determined to possess phytotoxic activity from fungal pathogens. Two novel C-glycosidic compounds from a bacterial plant pathogen were found to be the first low molecular weight elicitors of the hypersensitive response in plants. A verticillin of the epi-polythiodiketopiperazine class isolated from a soil fungus was discovered to be responsible for inhibition of ras farnesylation transferase. Leafcutter ants were found to be deterred by the presence of a novel sesqualterpenoid in epiphylls. Finally, inhibitors of the PDGF receptor were isolated and characterized from a lichen.