The complete life cycle of the trematode Ascocotyle (Phagicola) longa (Digenea: Heterophyidae) is elucidated by natural observation validated by experimental infections. The natural first intermediate host of A. (P.) longa, an agent of human heterophyiasis in Brazil, is the cochliopid snail Heleobia australis (new first intermediate host). Metacercariae were found encysted in the body musculature, heart, stomach, liver, kidney, spleen, gonads and mesentery of mullets Mugil liza. Hamsters Mesocricetus auratus were experimentally infected with metacercariae of A. (P.) longa obtained from the mullets, and the adults recovered were used to infect the snails H. australis. Rediae and cercariae of A. (P.) longa are described for the first time. The ultrastructure of the tegument of A. (P.) longa shows a change in spination pattern from the cercaria with single-pointed spines to the metacercaria and adult with multipointed, brush-shaped spines. The life cycle of A. (P.) longa is related to estuaries and coastal lagoons where the recruitment of mugilid juveniles occurs. The high prevalence (100%) of A. (P.) longa encysted in the mullets examined within the urban area of Rio de Janeiro indicates the potentially great public health impact of the consumption of raw mullets.