The 'bald'(bld) mutants of filamentous bacterium Streptomyces coelicolor A3(2) are characterized by a lack of aerial myceliumand spores. A 'bald' mutant was isolated exhibiting unusual features. It forms slightly sculptured colonies producing a red-orange mycelial pigment, large amounts of agarase and methylenomycin A; it is also highly resistant to U.V. killing. The bld mutation (bld F1) never reverted to bld+ phenotype and was localized very closed to met A.