The purpose of the present study was to compare the in vitro effects of a mechanical and a sonic toothbrush on the viability of Actinomyces viscosus, the rationale being that induction of irreparable microbial damage resulting from aggressive mechanical action or sonic energy, may inhibit or disrupt the process of successional colonization. Cultures of A. viscosus were grown to a standardized optical density and subdivided into 3 treatment groups of 20 specimens each. Treatment groups consisted of an untreated control and exposure to a mechanical or sonic toothbrush for 15, 30, 45, and 60 s. Subsequent to the prescribed treatment, samples were taken from each specimen dish, subcultured, and the number of CFUs determined. Additional samples were obtained for negative staining and examination by electron microscopy. The mean number of CFUs for each treatment group at each treatment interval were statistically analyzed by ANOVA and multiple pairwise comparisons. Results showed a significant main effect for toothbrushes (p<0.0001) and exposure time (p<0.01), but only marginal significance for the interaction of toothbrush with exposure time (p<0.055). Post-hoc tests showed a significantly greater number of CFUs for the sonic toothbrush compared to both the untreated control and mechanical toothbrush groups. Electron microscopic examination revealed a decrease in aggregation tendency and loss of fimbriae in the sonic toothbrush group. Based on the lack of morphologic evidence that would indicate cell damage and the increase in CFUs over that of the control group, it appeared that neither the mechanical or sonic toothbrushes affected cell viability.