Abstract The essential oils obtained from Satureja montana L. harvested in central part of Dalmatia (Croatia) at three ontogenical stages were evaluated for their chemical composition and antimicrobial activity. GC/MS analyses revealed the presence of 33 compounds in the oils. Carvacrol (52.4>26.2>16.1%) was found to be the main constituent especially before flowering while p-cymene (3.8<15.2<25.6%) increased through flowering. The antimicrobial activity of S. montana oils against five Gram-negative bacteria (Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, Pseudomonas aeruginosa, Salmonella typhi), four Gram-positive bacteria (Bacillus subtilis, Bacillus cereus, Staphylococcus aureus, Streptococcus faecalis) and five pathogenic fungi (Aspergillus fumigatus, Aspergillus niger, Candida albicans, Candida rugosa and Saccharomyces cerevisiae) was evaluated using the agar-plate dilution assay. The oil showed the strongest activity against all tests strains, with the exception of Pseudomonas aeruginosa. The maximum activity was observed against E. coli, S. aureus and fungal organisms. Escherichia coli had the lowest MIC of 0.06% with oils produced before and during the flowering period. These results support the notion that S. montana oil has a role as both a pharmaceutical and a preservative.