A sensitive dye-binding assay was employed to study the hyaluronidase associated with temperate and virulent phages infected group A streptococci. Some enzyme was detectable in each purified phage preparation examined, but differences of several orders of magnitude separated the lower enzyme levels in virulent phages that required the addition of hyaluronidase for plaque formation and the higher levels in temperate phages that did not. Infection by virulent phage A25 was accompanied by the production of levels of hyaluronidase proportionate to the average burst size. Hyaluronidase was produced during infection by temperate phages at a much higher level than could be accounted for by the number of phage particles formed. The major portion of this hyaluronidase was free and apparently unassociated with phage or phage fragments. The phage-associated enzyme was tightly bound but could be released and solubilized by treatment with urea.