Abstract The antimicrobial activity of newly synthesized linear monoazaphenothiazine derivatives; 3-benzamido-1-azaphenothiazine, 3-trifluoromethamido-1-azaphenothiazine, 3-trichloromethanamido-1-azaphenothiazine and 3-(4-nitrobenzamido)-1- azaphenothiazine have been evaluated against bacteria and fungal strains. Results showed that most of the synthesized linear monoazaphenothiazine derivatives were significantly active against the tested microorganisms. The correlation between antimicrobial activity and the chemical structure of phenothiazines was discussed.