Metabolites from 796 isolates of aerobic actinomycetes were screened for plant growth regulatory properties using an algal bioassay. These included 266 isolates ofStreptomyces, 28 unidentified actinomycetes, and 502 isolates of “novel” actinomycetes represented by 18 genera. Algal growth inhibition of ⩾30% was observed with 60 isolates, 37 of which belonged to the genusStreptomyces. Among other inhibitors were 8 isolates ofActinomadura, 6 ofActinoplanes, 2 each of the generaThermomonospora, Streptoverticillium, andPromicromonospora, and 3 unidentified. Metabolites from 70 isolates promoted algal growth by ⩾20%. These included 13 isolates ofMicromonospora, 11 ofStreptomyces, 6 ofNocardia, 5 ofActinomadura, and 4 each ofRhodococcus andThermomonospora. Sixteen unidentified isolates; 3 isolates ofPromicromonospora; 2 isolates each ofActinoplanes, Streptosporangium, andOerskovia; and 1 of “Thermoactinomyces peptonophilus-like” organism andSaccharomonospora viridis also promoted the algal growth by ⩾20%. The plant growth inhibitory properties of 9 actinomycetes and the growth promoting properties of 6 were demonstrable during the secondary screening on higher plants using chemicals extracted from the culture broth. The metabolites fromMicromonospora, Nocardia, Rhodococcus, Streptosporangium, andOerskovia isolates were associated with plant growth promotion only; those fromStreptomyces were most frequently involved with the growth inhibition.