In Iran, more than 100 humans and many livestock species have shown to be infected in the northern province of Mazandaran, at the Caspian Sea shore. This picture suggested the need for further multidisciplinary studies to ascertain the extent of the problematics. Three species of lymnaeids were found throughout most of Mazandaran: Lymnaea (Stagnicola) palustris, a secondary intermediate host of Fasciola hepatica; Galba truncatula, the main intermediate host of F. hepatica; and Radix gedrosiana, a member of the auricularia complex transmitting F. gigantica. Environmental microhabitat requirements are different for the three species, but sometimes the three were found in the same locality. The geographical distribution of the three lymnaeids agrees with the distribution of human and animal fascioliasis, differences in fascioliasis prevalences between western and eastern Mazandaran not appearing to be related with lymnaeids. Moreover, the overlapping distributions of G. truncatula and R. gedrosiana also agree with the overlapping distributions of F. hepatica and F. gigantica detected in Mazandaran animals. Although the three lymnaeid species may be found in all the months, interesting differences were detected. Whereas L. (S.) palustris appears to be more or less stable throughout the year, with only one acrophase around June, G. truncatula and R. gedrosiana showed an evident biseasonal distribution, with abundance and population density peaks in spring-beginning of summer and autumn. Such lymnaeid population biseasonality suggests a higher contamination risk by the two fasciolid species for both humans and animals in these two seasons and the corresponding detection of an increase of acute human cases attending the health centres. However, lymnaeid population dynamics do not explain the main peak of human diagnosed patients in the February-March period in Mazandaran. Peculiar culinary habits, as the custom of producing and stocking the traditional "delar" and its use as condiment sauce for many vegetables and food, may explain this chronological disagreement, tacking into account the long viability that fasciolid metacercariae can reach.