The length-weight relationship and condition factor (K) are useful descriptors in population fish biology, because they provide essential information on growth strategies, nutritional status and reproduction. They are also parameters widely used to compare the condition of aquatic systems with varying degrees of human intervention. Therefore, understanding the dynamics of populations in pristine ecosystems, provides relevant basic information to understanding changes or effects in populations under these pressures. The San Pedro river (Valdivia basin), southern Chile, is a system characterized by its current low intervention level, high fish diversity and low abundance of introduced species. We describe the length-weight relationship and condition factor of 12 native fish of the San Pedro river, for over two annual cycles, analyzing their growth strategies and seasonal variability in condition factor. The results of the length-weight show that the most of species exhibit isometric growth, especially Diplomystes camposensis, Basilichthys australis and Brachygalaxias bullocki, however, some species have a tendency to allometry (Galaxias maculatus, Galaxias platei and Trichomycterus areolatus). The values of K show that species such as Percilia gillissi have a strong annual variability, associated with the breeding season and greater supply of food. We give this information as baseline data to understand the functioning of fish populations in natural conditions and to be used for the evaluation of future projects that may impact these ecosystems.