The objective of this study was to assess the similarities and differences between selected bioindicator groups within tallgrass prairie and adjacent aspen forest. Based on pitfall trapping from 1998 to 2000, species richness and abundance of ground beetles and spiders were examined across a tallgrass prairie – aspen forest ecotone located near Winnipeg, Manitoba. The abundance of spiders and ground beetles was higher in the forested portion of the transect than in the prairie section. There were 639 specimens of ground beetles collected, representing 53 species; 19 species were found only on the prairie, 19 species were found only in the forest, and 15 species were found in both habitats, although the abundance of most species was too low to confidently assign them to either habitat type. Excluding single records in any sampling year yielded 9 prairie species, 5 forest species, and 10 species found in both aspen forest and tallgrass prairie. The five most abundant species of ground beetles were Agonum placidum (Say), Pterostichus caudicalis (Say), P. femoralis (Kirby), P. melanarius (Illiger), and Synuchus impunctatus (Say). There were 4499 specimens of spiders collected, representing 92 species; 25 species were found only in the prairie, 15 species were unique to the aspen forest, and 52 species were found in both habitats. Excluding single records in any sampling year yielded 26 prairie species, 15 forest species, and 22 species found in both habitats. The five most abundant species of spiders were Pardosa moesta Banks, P. distincta (Blackwall), Agroeca ornata Banks, Centromerus sylvaticus (Blackwall), and Alopecosa aculeata (Clerck). Our study concurs with other studies in demonstrating that there are distinct assemblages of both groups of predators in each of the two habitat types.