In a study of the digeneans parasitising molluscs in the Els Alfacs lagoon (Ebro Delta, Western Mediterranean) we found heavy infections with sporocysts emitting two types of cotylocercous cercariae in the prosobranch trochid gastropod Gibbula adansonii and with metacercariae in the prosobranch nassariid gastropod Cyclope neritea. A comparative analysis using ITS ribosomal DNA sequences from these larval stages and published sequences of 17 larval and adult opecoelid stages allowed us to elucidate the life-cycle of Macvicaria obovata and to confirm the identification of Cainocreadium labracis based on cercarial morphology. We provide molecular evidence for the identification and the first detailed morphological descriptions of the intramolluscan larval stages of the two opecoelid species as well as partial 28S rDNA sequences to aid future studies on systematic relationships within the Opecoelidae.