Interbacterial coaggregation between Actinomyces viscosus indigenous to the human mouth and Streptococcus pyogenes and Streptococcus agalactiae was studied. Fifteen of twenty-six strains of Streptococcus pyogenes and thirteen of thirty-one Streptococcus agalactiae showed a coaggregation with Actinomyces viscosus strain. The results show that the coaggregation mechanism required calcium and was dependent on pH. Some coaggregations were inhibited by 0.06 M. lactose and by 1 M. NaCl.