Up until now, there have not been many studies of single Gyrodactylus manipulations on salmonids performed in order to study the establishment, development and reproduction of the parasites. The results of the current thesis indicated a relatively marked ecological difference between Gyrodactylus salaris and G. thymalli with respect to host preference and population growth. However, a potential for co-occurrence of both species on the same host still exists. This study provides results from extensive infection attempts conducted on Norwegian Atlantic salmon (River Altaelva), Baltic salmon (River Neva) and grayling (River Trysilelva). All hosts were infected with G. salaris (both an Atlantic salmon and an Arctic charr strain) and G. thyamlli (River Trysilelva strain). The correlation between population growth of isolated salmon and grayling, and the survival and reproduction of the individual Gyrodactylus strains/species on different hosts, was studied. The results indicate that Norwegian Atlantic salmon are unlikely to be a functional host for the G. thymalli strain used in this study. Furthermore, the Arctic charr strain of G. salaris that was used for the first time to be infected onto grayling and could in contrast to on salmon successfully establish and give birth on grayling. This indicates that ecological host preferences are not reflected in the mitochondrial cytochrome oxidase I (COI) systematics. The Norwegian Atlantic salmon demonstrated variability in susceptibility and resistance to the pathogenic G. salaris strain. This was also observed for G. thymalli on grayling. The population reproductive rates (r) calculated for all parasite strains on the respective hosts showed a steady decline regardless of the initial parasite population size. This may reflect a fundamental process in gyrodactylid population biology, and the host may act against the parasite s presence or activity from the start of the infection. The comparison between theoretic and natural growth indicated that only G. salaris (Atlantic salmon strain) on Norwegian Atlantic salmon and G. thymalli on grayling followed the expected growth curves based on intrinsic rate of increase (rm) values, although for G. thymalli on grayling an acquired host immune reaction was noticed.