Saprolegnia parasitica and Aphanomyces astaci are oomycete pathogens that cause significant economic losses in salmonid and crayfish aquaculture. In this dissertation, inhibition of their growth and reproduction by sage, rosemary and bay laurel essential oils and fluid propolis formulations was investigated, as environmentally friendly alternatives to the toxic chemicals currently used. Considering the high cost of essential oil production, the isolation of essential oil from bay laurel, sage and rosemary leaves was optimised to increase the yield. Ultrasonic and reflux extraction hydrodistillation pretreatments increased the essential oil yield by up to 60%, while the overall quality of the oil remained largely unchanged. In contrast, the use of cell wal degrading enzymes as hydrodistillation pretreatments did not affect the yield. Next, it was shown that the tested essential oils and propolis formulations strongly inhibited the motility and germination of zoospores and the growth of the mycelium of both pathogens, albeit with differences in efficacy depending on the substance, oomycete species and life cycle stage. The fluid propolis formulations most potently inhibited mycelial growth of A. astaci, while the sage essential oil most strongly inhibited zoospore germination and mycelium growth of S. parasitica and zoospore germination of A. astaci. Although the results point to several compounds as the basis of the inhibitory effect, such as camphor in sage and chrysin in propolis, the observed anti-oomycetic activity is probably due to the synergistic effect of many different bioactive components present at low concentrations. Overall, the results of this doctoral thesis provide a starting point for further in vivo testing of the use of essential oils and propolis formulations in freshwater aquaculture as an environmentally acceptable control of diseases caused by oomycetes.