Some of the most devastating plant and animal pathogens belong to the oomycetes. Saprolegnia parasitica, a major pathogen of freshwater fish and the causal agent of Saprolegniosis, causes large economic losses in aquaculture due to the death of freshwater fish which contract Saprolegniosis. Losses due to Saprolegniosis are also incurred by natural fish populations. Saprolegniosis was previously controlled through the use of a biocide called malachite green, until the dye was found to have potentially carcinogenic effects. Following this discovery, the use of malachite green on fish destined to enter the food chain was banned worldwide and subsequently there has been an increase in the occurrence of Saprolegniosis. This work investigates various aspects of S. parasitica with the view to inform the development of new control strategies. Three cDNA libraries, including two interactive libraries, were analysed and bioinformatics provided valuable insight into the genes expressed not only by S. parasitica but also by the host. The annotation and expression profiles of several of these genes are described in this work and the potential of S. parasitica genes as vaccine targets is discussed. One protein, identified as a vaccine candidate, is annotated, cloned and overexpressed in order to evaluate its efficacy as a vaccine. It has been shown previously that the plant pathogenic oomycete Phytophthora infestans is able to utilise a protein localisation motif, the RxLR, in order to translocate effector proteins into the host. The question as to whether S. parasitica possesses a similar translocation system is addressed in this work. Localisation studies on an RxLR protein of S. parasitica origin are described and the microscopic interaction between S. parasitica and its host are described and attempts to optimise production of interaction structures are discussed.