In this study, usability of plant essential oils from Lamiaceae species for Aeromonas salmonicida infections was investigated under in vitro and in vivo conditions. A total of 18 A. salmonicida strains were obtained from the organs and tissues of locally cultured rainbow trout. All strains were 100% resistant to penicillin, trimethoprim and bacitrasin. All strains showed widest resistance profile to antibiotics. Antibacterial effects of essential oils obtained through water distillation of aerial parts of 4 Lamiaceae species (Origanum onites, Origanum vulgare sp. hirtum, Thymbra spicata var. intricata and Satureja thymbra) were high on 17 out of 18 strains and inhibition zones ranged between 14-25, 12-26, 10-30 and 10-33 mm, respectively but none of them was effective on A. salmonicida FC84 strain. The most sensitive bacteria to Origanum essential oils was A. salmonicida FC29 strain and the most effective essential oil was S. thymbra's. Therefore, MIC value of this essential oil on FC29 strain was determined. Then, the MIC value (800 mu g) of this esential oil were injected to experimental fishes for in vivo studies. Injection of the most effective dose of S. thymbra essential oil and its several dilutions (400, 200, 100 mu g mu L-1) caused toxic effect and total mortality of experimental fishes. The essential oil at the doses with low or no toxic effect did not increase the bactericidal activity of fish blood indicating that it does not protect rainbow trout against A. salmonicida infections. On the other hand, S. thymbra essential oil at non-toxic low dosages can be used as immune stimulant.