An effective alternative was discovered in the form of thyme oil for use as a fish anaesthetic (patent pending approval). The thyme oil along with a common aquaculture-grade commercial anaesthetic (AQUI-S), clove oil and quinaldine were investigated for their antimicrobial properties and its effect on the immune parameters of two important maricultured fish species, bluefin bream (Sparidentax hasta) and yellowfin bream (Acanthopagrus latus). In vivo studies indicated that both the fish species had highly reduced bacterial load after the treatments and the in vitro antibacterial activity of the of the thyme oil was superior to that of the other treatments. The effects of anaesthetic dose of thyme oil, clove oil, quinaldine and AQUI-S were evaluated and compared. The reduction in the total viable vibrio counts in the anesthetized fish indicated that the vibrio were sensitive to the thyme oil. Also thyme oil produced higher non-specific immune enhancements.