The presence of trematodes in fish products is a problem of concern to health authorities because of the potential hazards. Viability studies of heterophyid metacercariae are still scarce and lack methodological standardization. In this study of the heterophyid trematode Ascocotyle (Phagicola) longa, we isolated metacercariae from Mugil liza and conducted a controlled experiment to test the effects of heating or freezing mullet musculature on the survival of the metacercariae. The parasites were incubated in mullet muscle tissue at different temperatures, ranging from 20 C to 180 C, for different intervals. The physical damages were evaluated through SEM images. The estimated number of metacercariae in mullets was 514 in 308 g of musculature. Control metacercariae (no freezing or heating) were highly motile and showed no external damage after excystation with trypsin. All metacercariae were dead after heating for 15 min at 60 C, 100 C and 180 C. For freezing temperatures, all metacercariae died after two hours of exposure to 35 C and 20 C, but 24 h of exposure to 10 C was necessary to kill all metacercariae present in the fillets. The SEM images showed damage to the tegument of A. (P.) longa exposed to heating and freezing temperatures that included some cracks and areas of spine loss. Parasites exposed to extreme temperatures also seemed to shrink. According to our results, the recent update of Brazilian law on food safety with specific procedures for inactivation of parasites was important to prevent outbreaks of trematodiases caused by Ascocotyle (P.) longa.