The transmission of the nematode Hysterothylacium aduncum to seawater netpen-cultured salmonids in Chile was studied in order to clarify the life history of this enteroparasite in marine farms and to propose a control strategy. The stomach content analysis of salmonids revealed that trout Oncorhynchus mykiss which are most liable to carry H. aduncum, feed considerably on natural organisms, less so with coho salmon O. kisutch and almost not at all in Atlantic salmon Salmo salar which are not infected. In trout, the most important food item is gammarids followed by stomatopod larvae. Polychaetes and zooplanktonic anomurans are also present. Gammarids from nets and floats of cages seems to be mainly responsible for nematode transmission. The in vitro culture and experimental infection of the nematode demonstrated that the L3 that hatches from the egg easily infects their first intermediate host a calanoid or harpacticoid copepod. In this host the larvae develop to the typical form observed in salmonids and other intermediate or paratenic hosts. From two to four-host life cycles are proposed. A systematic removal of netpen fouling organisms is necessary to eliminate the intermediate hosts and prevent the transmission of the nematode to the salmonids. The prevalence and abundance of the nematode in salmonids are similar to those found in the wild for their natural and definitive host Merluccius australis.