It was assessed the parasitic relationship between by 2 morphospecies of microphallid metacercariae and their second intermediate host, the amphipod Apohyale hirtipalma with the body length and sex of the hosts, and the abundance of shore birds (definitive hosts). Amphipod mortality induced by the parasites was assessed through the relationship between the variance/mean parasite abundance ratio with host body length for each metacercariae morphospecies. Four sites at the Biobío Region (36°S), Chile, were selected because of contrasting abundance of potential definitive hosts (Aves: Laridae), from where 240 amphipods were collected from intertidal pools (60 in each sampling locality). Total prevalence was 57.5% and metacercariae found, belonged to 2 unidentified morphospecies of microphallid subfamilies (Microphallinae, 28.3% and Maritrematinae, 36.7%). The prevalence and abundance of metacercariae increased with the amphipod body, although their variations were associated to the abundance of shorebirds. The prevalence of Microphallinae did not have differences among sexes in their relationship with the host body length, while among Maritrematinae, male amphipods showed higher prevalence than females of similar body length. It was also determined that Maritrematinae would cause more mortality than Microphallinae. Local variations of the abundance of other hosts involved in the microphallid life cycles as well as of prevailing local environmental conditions and amphipod behavior may be keys for a better understanding of the abundance and prevalence variations of these parasites.