The effects are described of three different media, based on urea, glycine or meat casein digest as nitrogen sources, on the morphology of Streptomyces griseus when grown in submerged culture. The urea medium was characterized by an alternation of morphological generations similar to that described by Klieneberger-Nobel for surface cultures: ‘primary mycelium’ alternated with ‘secondary mycelium’, the two generations being separated by an ‘initial cell’ stage. ‘Secondary mycelium’ sporulated to give ‘secondary mycelium spores’. The other two media did not support ‘initial cell’ formation, but led either to the development of intercalary ‘chlamydospores’ or lysis without sporulation. The media based on urea or glycine were used in convenient and satisfactory techniques for the production of ampoules containing large numbers of lyophilized spores which stored satisfactorily.