Biosynthesis of the macrolide antibiotic tylosin was studied in fed-batch fermentations. When glucose and sodium glutamate were fed to the fermentation in a cyclic square wave fashion with a period of 24 hours, it was possible to maintain high initial rates of tylosin biosynthesis for up to 240 hours before the synthetic rate decreased. By adding basic salts to the solutions used for the cyclic feeding, it was possible to extend the period of tylosin biosynthesis until 480 hours.