The increasing impact of some parasitoses on fish health and its economic relevance in aquaculture and fisheries has enhanced the need for studies on piscine parasites and on the fish-parasite relationships. Fish parasites are an important part of water ecosystems, and thus, changes in these habitats can affect the host-parasite equilibrium. The research teams leaded by the author have studied piscine parasites in Spanish waters since 1970. In this Part 1, the parasite fauna of continental fish (mainly salmonids and cyprinids), mostly collected in rives of the Duero basin (NW Spain) from 1970 to 1984, is presented, including 62 figures. After a brief reference to the pathologcal concern of some fish parasitoses, the population dynamics of the found parasites is analysed. The helminth fauna of macroinvertebrates of some rivers is also reported in relation to the helminth life cycles. Some recent findings on this filed in Spain and in other European countries are also commented and compared. This book is useful for students or graduates in Bological Sciences and Veterinary Medicine, and also for ecologists or ichthyologists interested in the role of parasites in aquatic habitats.