Oomycetes are eukaryotic fungus-like microorganisms that are known to be pathogenic to plants and animals, causing both ecological and economic damage. Animal pathogenic oomycetes, such as Saprolegnia species are destructive pathogens to many aquatic organisms and are found in most parts of the world. Phytophthora species cause a number of plant diseases. Pythium are less understood as many of these species are saprotrophs which are not thought to be pathogenic. Oomycete research has focussed mainly towards the characterisation and control of pathogenic species with very little information on the ecology of Oomycetes. In this thesis, oomycete species were isolated from water samples collected from rivers in Scotland and Northern England. Baiting and water filtration were used to compare the reliability of both methods, and nested PCR was used to compare the number and types of species obtained in comparison to conventional culturing methodologies. The results suggest that water filtration provided a fast, reliable method for isolating abundant, hardier species such as Pythium undulatum, Saprolegnia diclina, Saprolegnia delica, and Saprolegnia ferax. Water chemistry, surrounding land use, seasonality and land elevation were found to be important factors in Oomycete diversity. Nested PCR of oomycetes directly from filters to obtain target DNA from organisms which are more difficult to culture or less numerous proved to be useful for some species, but will need more refinement of primer and methodologies to obtain species of interest. Sampling of the rivers resulted in the isolation of three new Pythium spp. isolated from the Rivers Spey and Dochart in Scotland. Phylogenetic analysis, infection studies, growth rates and microscopy were used to characterise these three species.