Streptomyces lusitanus is known to produce chlortetracycline in the presence of Cl- and tetracycline in a medium without Cl- (Neth. 266,725). A new strain has been isolated which produces only tetracycline, even in the presence of Cl-. It has been named S. luistanus var tetracyclini, and by careful selection a mutant 106-T NCIB 9500 has been obtained. By fermentation this strain produces 9-12 g. tetracycline/l. medium. Details of growth and culture properties are given. The strain does not grow if only inorg. N is given. By addn. of up to 65 g. starch hydrolyzate/l. of medium, the yield is enhanced. Another promotor is N,N'-dibenzylethylenediamine in a quantity of 0.1-1 g./l. Fermentation and isolation of the antibiotic are carried out in the usual manner.