Upper Middle Ordovician carbonate rocks of the Trenton Group between Montreal and Quebec City in the St. Lawrence Lowland, eastern Canada, contain a moderately diverse and abundant trace fossil assemblage. Although the trace fossils represent a variety of behavioral groups, the rarity and/or poor preservation of some of the forms has prevented definite identification. The following have, nevertheless, been recognized: Arenicolites, ?Calycraterion, Chondrites spp., Circulichnis montanus, Clematischnia, ?Conostichus, Cruziana problematica, Cruziana spp., cf. Diplichnites, Furculosus carpathicus, Helminthopsis hieroglyphica, Helminthopsis, Oichnus paraboloides, Palaeophycus tubularis, Palaeophycus, ?Plagiogmus, Planolites beverleyensis, Planolites montanus, Planolites, ?Rhizocorallium cf. R. irregulare, ?Rosselia, Scalarituba missouriensis, Skolithos linearis, Skolithos, Teichichnus rectus, Teichichnus, Trichichnus, Trypanites weisei, Vermiforichnus clarkei and cf. Zoophycos as well as informally diagnosed loop, oblique and pronged burrows and bryozoan borings. Of these forms, only Chondrites spp., Palaeophycus tubularis, Palaeophycus, Planolites spp., Teichichnus and Trypanites weisei are abundant; the remainder are rare to only moderately common. Sediments of the Trenton Group were deposited initially in lagoons followed in turn by offshore bar, shallow and, finally, deeper offshore shelf environments. The trace fossils do not exhibit significant variation with respect to these broad depositional regimes and, instead, each environment is characterized by assemblages typical of the Cruziana ichnofacies as recognized in clastic sequences. Although both early and late diagenesis caused important modification of the trace fossils, it did not mask their original environmental distributions.--Modified journal abstract.