Metabolites from 906 microbial isolates were evaluated for herbicidal properties. These included 266 isolates of Streptomyces, 502 isolates of non-Streptomyces actinomycetes 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 the Streptomyces and Nocardiopsis isolates and 13% of Actinoplanes isolates were toxic to cress (Lepidium sativum L.) seeds. Among other inhibitors were three isolates of Actinomadura and one isolate each of Micromonospora, Micropolyspora, Strep to sporangium, Streptoverticillium, and Bacillus, and two isolates of unidentified actinomycetes. The toxigenic fungi included two isolates of Penicillium and one isolate each of Aspergillus, Scopulariopsis, and Paecilomyces species. About half the isolates toxic to cress were toxic to barnyardgrass (Echinochloa crus-galli L. Beauv. # ECHCG) seeds. of the isolates found effective in the secondary screening on the potted weeds, eight belonged to the genus Streptomyces and one was identified as Scopulariopsis brumptii. None of the examined isolates of Rhodococcus, Nocardia, Oerskovia, Thermoactinomyces, Thermomonospora, and six other genera of actinomycetes showed any appreciable toxicity to the seeds of either species.