The Lomatium triternatum complex is widespread in the Columbia River Basin. Lomatium triternatum (Pursh) J. M. Coult. & Rose var. triternatum and L. triternatum (Pursh) J. M. Coult. & Rose var. anomalum (M. E. Jones ex J. M. Coult. & Rose) Mathias are sympatric throughout much of their range and are reported to have similar fruit but different leaves. Plants of the two varieties repeatedly occur within 5–30 m of each other at an unusual site in northwest Montana, USA. We collected data on leaf and fruit morphological characters and habitat associations at this site to help address whether the proper taxonomic rank for these two taxa should be revised. Terminal leaflet shape and specific leaf area differed between the two varieties with little or no overlap. Fruit mericarp length and width also differed between the two varieties. These patterns are not consistent with treating L. triternatum var. triternatum and L. triternatum var. anomalum as sympatric, interbreeding, conspecific taxa. The two varieties of L. triternatum also occurred in different plant communities in spite of growing in close proximity: var. anomalum occurs with tall, leafy forbs, while var. triternatum is associated with xeric-adapted bunchgrasses and cushion-forming forbs. We conclude that the two varieties are better recognized as separate species.