Monogeneans are common aquatic ectoparasites within the phylum Platyhelminthes. Freshwater monogeneans have been little studied in New Zealand, with only three taxa having been recognised in the country until recently, including only two to species level. Twenty-one non-native freshwater fish species have established populations in New Zealand, yet only one of these has been reported with monogenean infestations. Based on the diversity of monogenean species known from these fish species globally, it is likely that many monogenean species will have established populations in New Zealand that remain unreported. The prevalence and intensity of parasite infection and infestation are known to vary with season and in relation to host factors such as size and sex, but little is known of their ecology in New Zealand freshwater fish species. I report for the first time the presence of eleven monogenean species infesting non-native freshwater fish in New Zealand. Two of these species belong to the family Ancyrocephalidae, a family new to New Zealand. A seasonal study of two species infesting Gambusia affinis (Baird & Girard, 1853) in two interconnected ponds in Hamilton, Salsuginus seculus (Mizelle & Arcadi, 1945) and Gyrodactylus gambusiae Rogers and Wellborn, 1965, represents the first ecological study of monogeneans undertaken in New Zealand, a country with a mild climate and narrow annual temperature range relative to better studied northern temperate populations. The prevalence of S. seculus changed significantly over the course of a year, reaching a peak during the summer. However, no significant correlation with temperature was found for either the prevalence or mean intensity of S. seculus. Prevalence was instead correlated with the concentration of chlorophyll a and pH. However, it is likely these correlations were simply due to chlorophyll a concentrations similarly increasing over summer affecting the pH, rather than either variable being causative. As G. affinis is known to experience large increases and decreases in population density annually, with individuals coming into physical contact during breeding, this was likely the cause of the increase in monogenean prevalence. A greater number of hosts would provide more habitat for monogeneans and their increased density would facilitate transmission of the parasites. The sex of fish was found to be a significant predictor of prevalence and intensity of monogeneans S. seculus, but this was on the basis of size. Longer and heavier fish had higher prevalence and mean intensity of S. seculus, and as female G. affinis are typically larger than males, this led to the difference in infestation between the sexes. Larger individuals represent larger habitat patches and for this may be the reason that prevalence and intensity of monogeneans was higher on these individuals. A similar test was conducted on Carassius auratus (L.), where two size classes were observed, representing juveniles and adults. The juvenile fish had significantly higher prevalence and intensities of Dactylogyrus spp., indicating a parasite vulnerability in juveniles. This thesis contributes to the knowledge of the species diversity and ecology of freshwater monogeneans in New Zealand.