Owing to the veterinary and medical importance of heterophyid trematodes, a survey on Ascocotyle (Phagicola) longa in different organs of mullets Mugil liza from Rio de Janeiro was undertaken. The prevalence of metacercariae varied greatly between different organs of the mullets: spleen (100%), heart (98%), intestine wall (97%), liver (97%), muscle (87%), stomach wall (77%), brain (47%), gonads (30%) and gall bladder (30%). The high level of the intensity of the infection in relation to different fish organs was confirmed in two experimental infections performed during the spring/summer and autumn/winter seasons when 258 and 47 adult parasites were recovered from hamsters fed only with small pieces of muscle tissue. The potential risk of infection was considered to be high in view of the high prevalence and intensity of A. (P.) longa in the muscles of mullets throughout the year. Additionally new confocal imaging of metacercariae and adults experimentally obtained, enabled for the first time the description of a short genital atrium formed by the union of uterus and ejaculatory duct.