The ability of humanActinomyces naeslundiistrains to aggregate with dental plaque streptococci was found to be strain specific. A.naeslundiistrains aggregated strongly more often with specific strains ofStreptococcus sanguisandStreptococcus mitisthan withStreptococcus mutansstrains. The effects of proteolytic enzymes and heat on the interbacterial affinities ofA. naeslundii398A andS. sanguisS, two strains which were found to aggregate strongly, were studied. Enzyme or heat treatment of 398 A, but notS. sanguisS, impaired aggregation. Electron microscopic examination of aggregates of untreated bacteria revealed that the attachment between cells of strains 398 A and S was mediated by short tufts of electron‐dense fuzzy components of the cell surfaces. The affinity between cells ofA. naeslundiiand the streptococci which readily colonize cleaned smooth tooth surfaces may account, in part, for the previously‐reported, delayed increase inA. naeslundiiproportions among bacteria forming early plaque depos