The characterization of Prosorhynchus crucibulum (Rudolphi, 1819) Odhner, 1905 egg and miracidium is important in order to better understand the transmission dynamics between the definitive host and the primary host, the mussel. In this way, the objective of this work was to study the miracidia morphology, in order to evaluate if this species belongs to the group of passive or active transmission larvae. The morphology of eggs is similar to the ones presented by other Prosorhynchus species, with a small size of 26 × 17 μm, and four-five times smaller than the ones of Fasciola hepatica. The number of eggs produced per worm was around 6,760 (4,236-8,401), which was four-five times higher than in F. hepatica. The miracidia presented small dimensions 24 × 15 μm (23-25 × 13-15 μm range), a long stylet, two ciliated epithelial plates, very long cilia (12.7 μm) and absence of terebratorium and eyespots. Those features of the miracidia suggest that P. crucibulum belongs to the group of passively infecting larvae.