As a result of a study of antimicrobial agents from Spanish Sideritis species, we now report a series of natural and semisynthetic diterpenoid compounds with selective antimicrobial activity toward Gram-positive bacteria and, in some cases, also toward yeasts. A 1,3diolic group is responsible for this activity. Another hydroxyl group on the molecule produces a drastic decrease in antimicrobial activity. On the other hand, an acetoxyl group increases the inhibitory action. To the best of our knowledge, 1-acetyljativatriol (2) is the first natural entbeyerenic diterpenoid which has been proved to show antimicrobial activity.