METACERCARIAE O F TREMATODES PARASITIZING FRESHWATER FISH IN MEXICO: A REAPPRAISAL AND METHODS OF STUDY TomáS Scholz & María Leopoldina Aguirre-Macedo Metacercariae of trematodes that represent the most abundant and frequent component of the helminth fauna of freshwater fish in Mexico are briefly reviewed. Of 49 species recorded, 25 have been identified to species level. Most metacercariae (39 species) have been found in cichlid fish from southeastem Mexico, whereas fish species of other families harbour fewer trematodes. Methods for study of metacercariae of fish are described to facilitate further research in Mexico. It is proposed that these studies include the following steps: (i) examination of fresh fish; (ii) observations of metacercariae in sitzi (within cysts) and in vivo after their isolation from cysts, including measurement and illustration; (iii) fixation of metacercariae with ammonium picrate for semipermanent preparations and with 4% hot formaldehyde solution for biometrical studies and staining; (iv) experimental infections of potential definitive hosts to obtain adults for species identification, and (v) evaluation of the impact of metacercariae on fish hosts. INTRODUCTION In Mexico, metacercariae of trematodes represent the dominant component of communities of helminth parasites (Scholz et al. 1995b; PBrez Ponce de León el al. 1996; Salgado-Maldonado et al. 1997; Salgado- Maldonado & Kennedy 1997). Some species, such as Oligogonotylus manteri Watson 1976, Echinochasmus leopoldinae Scholz, Ditrich & Vargas-Vázquez 1996, Posthodylostomum minimum (MacCallum 1921), and Ascoco&le (Phagicola) nana Ransom 1920 occur in high numbers and may negatively affect the health of heavily infected fish (Osorio-Sarabia et al. 1986a; Aguirre-Macedo 1990; ~imá-Álvarez et al. 1994; Scholz et al. 1995b). Despite the importante of trematode metacercariae as the most abundant fish parasites and potential pathogens, there are still considerable gaps in the knowledge of species identity and life cycles of most species occurring in freshwater fish. Salgado-Maldonado, G., A.N. Garcla Aldrete and V.M. Vidal-Martinez (Editors). 2000. Metazoan parasites in the neotropics: a systematic and ecological perspective. Instituto de Biologia, UNAM. México. p. 101-1 15.