A key issue in Evolutionary Developmental Biology is to assess the roles of homeotic genes in order to uncover the origins of animal diversity. Within parasitic platyhelminths which show a large diversity of developmental strategies, only one study related to the expression of Hox genes has so far been conducted involving a digenean species with a complex life cycle. In the present study, we considered the expression levels of the Pg-Lox4 gene within Polystoma gallieni of the Monogenea which displays alternative phenotypes throughout its direct life cycle, depending on the physiological stage of its amphibian host Hyla meridionalis upon which free swimming larvae attach. Dissimilar expression patterns were found along the two morphogenetic routes revealing a putative role of Pg-Lox4 in the process of developmental plasticity. Pg-Lox4 was also shown to be upregulated in both reproducing parasite phenotypes indicating its apparent involvement in tissue differentiation of the reproductive organs.