Purpose:Assessment of streptococcus mutans and lactobacillus adhesion to zirconia and stainless-steel crowns in primary molars Methods: One hundred and twenty randomly selected Egyptian children received bilateral crowns to first or second primary molars, one side was restored by stainless steel crowns (group A) and the other side was restored by zirconia crowns (group B). swabs were collected before preparation of crowns, 1 month and 3 months after cementation Results: There was no significant difference in the count of streptococcus mutans and lactobacillus before preparation of zirconia crowns, after 1 month and after 3 months. There was significant difference in the count of streptococcus mutans and lactobacillus before preparation of SSCs, after 1 month and after 3 months Conclusions: Zirconia crowns presented to be an excellent choice for primary posterior teeth full coverage restorations. Zirconia crowns performed better than SSCs in the aspect of prevention of plaque adhesion.