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In the last years, a growing number of studies have focused on the search for biological indicators of human impact in freshwater environments. Given their susceptibility to different types of impact, larval digeneans have been considered as useful tools for the study of temporal variations in parasite communities. However, few studies have been carried out in lotic environments. For that reason, the spatio-temporal variations of the larval digenean assemblages parasitizing the gastropod Heleobia parchappii were analysed in four human-impacted streams, located on the coast of Argentina. In total, 4800 specimens of H. parchappii were collected seasonally, and 12 digenean species belonging to eight families were registered. Three of the streams showed similar species presence but, during spring and summer, the streams located within the city presented lower species presence than the streams located in the urban periphery. By contrast, the spatial prevalence values evidenced a higher variation between the environments. Streams located in the urban periphery showed higher prevalence values throughout the year, compared to those calculated for streams located within the city. The spatio-temporal variations of larval digenean assemblages parasitizing H. parchappii seem to be mainly influenced by the diversity and vagility of definitive hosts, the types of digenean life cycles and habitat characteristics. However, environmental disturbances derived from anthropogenic activities are highlighted as the probable main factors that may affect the composition and dynamic of these parasite assemblages. [ABSTRACT FROM AUTHOR]