Metabolites from 906 microbial isolates were evaluated for herbicidal properties. These included 266 isolates ofStreptomyces, 502 isolates of non-Streptomycesactinomycetes representing 18 genera, 28 unidentified aerobic actinomycetes, 70 fungi, and 40 isolates of eubacteria. Metabolites from 72 isolates significantly inhibited germination of cress seeds. In terms of generic specificity and frequency, about 18% of all theStreptomycesandNocardiopsisisolates and 13% ofActinoplanesisolates were toxic to cress (Lepidium sativumL.) seeds. Among other inhibitors were three isolates ofActinomaduraand one isolate each ofMicromonospora, Micropolyspora, Strep to sporangium, Streptoverticillium, andBacillus, and two isolates of unidentified actinomycetes. The toxigenic fungi included two isolates ofPenicilliumand one isolate each ofAspergillus, Scopulariopsis, andPaecilomycesspecies. About half the isolates toxic to cress were toxic to barnyardgrass (Echinochloa crus-galliL. Beauv. # ECHCG) seeds. of the isolates found effective in the secondary screening on the potted weeds, eight belonged to the genusStreptomycesand one was identified asScopulariopsis brumptii. None of the examined isolates ofRhodococcus, Nocardia, Oerskovia, Thermoactinomyces, Thermomonospora, andsix other genera of actinomycetes showed any appreciable toxicity to the seeds of either species.