To study the distribution of Lymnaea truncatula in the Porma river basin (León, NW Spain) and its helminth fauna, malacological samplings were carried out at 66 points in the basin and the presence of these molluscs was detected in 31. To trace the dynamics of this mollusc population and the prevalence and intensity of its infection by trematodes, malacological samplings were made at fortnightly intervals over two years at five locations, situated in the upper and middle regions of the river Porma basin. To confirm the identity of the larval stages found in the molluscs, second intermediate and/or definitive hosts, depending on the trematode species, were experimentally infected to complete the life cycles. Two different species of Plagiorchiidae and one of Notocotylidae were identified. The infection prevalence of Plagiorchis elegans for the 6291 specimens of L. truncatula examined was 2.8% and infection was observed in snails collected in 7 of the 31 sample localities. This parasite was found in all months of the year, with the highest prevalence observed in July and October. When the corrected frequency values were considered, a slightly positive relationship was observed between the infection prevalence and the latter months. The highest percentages of snails harbouring immature sporocysts were detected in March and June–July, while the highest percentages with sporocysts containing mature cercariae were observed in spring and at the end of summer–autumn, and cercarial shedding in the latter. Of the 6291 L. truncatula examined 0.3% were infected by sporocysts of Opisthioglyphe ranae. The infection was detected in only one locality, in four months of the year, with the highest prevalence in May. Notocotylus neyrai was found in 2.6% of the 6291 mollusc specimens checked and was collected from 12 of the 31 localities. The highest prevalence was observed in October, April and July and the corrected frequency values reveal a positive relationship between the infection prevalence and those three months. The highest percentage of molluscs harbouring rediae of N. neyrai with germinal mass was found in April and September–October. On the other hand, rediae with mature cercariae were present in almost every month, but we only observed cercarial shedding in October and March. The L. truncatula infection prevalence by P. elegans, O. ranae and N. neyrai increased with the size of the molluscs. Double infection was only observed in four snails: one harboured O. ranae and N. neyrai and three F. hepatica and P. elegans.