Erythromycin production in 1.5 l continuous cultures of Saccharopolyspora erythraea, was correlated with the mean mycelial fragment size, which was influenced by culture agitation (750-1 500 rev./min marine/disk turbine impellors). Kinetic parameters for hyphal breakage were determined, and revealed that more highly branched mycelial fragments (hyphal growth unit similar to 14 mu m, compared to similar to 21 mu m) were more resistant to breakage. Branching rate depended on growth rate, the more highly branched mycelium being observed at the higher dilution rate. The breakage rate was inversely related to the mycelial fragment diameter at all dilution rates. Manipulating the size distribution of mycelial fragment by using an ultrasonic filter improved specific antibiotic productivity by 33%.