Artículo de publicación ISICaligus rogercresseyi is a sea louse of native and farmed fish in southern Chile. Due to the impact that this copepod has on the Chilean salmonid production, it is necessary to search for control mechanisms that can effectively reduce its population. This study focuses on determining the feasibility of using Udonella sp., a common flatworm found on copepod ectoparasites of the Chilean rock cod, Eleginops maclovinus, as a biological control for sea lice. The following aspects were studied: distribution and preference of Udonella sp. regarding both host sex and body location on copepods collected from both the Chilean rock cod and cultured salmonids; the effect of Udonella on both fecundity and survival of the free-living stages; and seasonal variations of population of both Udonella and the copepod. Results indicated that Udonella sp. is frequently found on copepods collected from the Chilean rock cod, but is absent on those from cultured salmonids. The flatworm showed a marked preference for female copepods and the main body locations used by Udonella are the genital complex and egg sacs. The three life stages of Udonella are distributed across the copepod body in different ways since different body locations represent feeding and nursery areas. Udonella has no significant effect either on fecundity or survival of free-living stages. Udonella may maintain its population relatively independent of the Caligus infra-population all year round. Seasonal patterns of variation were not evident for the population parameters of either Caligus or Udonella, probably due to the exceptional environmental conditions that occurred during 1997 that allowed Caligus populations to remain high during fall and winter, instead of decreasing as it has been reported previously. Results suggest that Udonella cannot be used as a biological control for sea lice since it is not found naturally on salmonid species and because the presence of Udonella did not reduce either fecundity or survival of free-living stages of copepods.