A combination of observational and experimental methods, in both the laboratory and field, were used to assess niche partitioning between Ceutorhynchus alliariae Brisout and C. roberti Gyllenhal (Coleoptera: Curculionidae), two coexisting shoot-boring weevils on garlic mustard, Alliaria petiolata (M. Bieb.) Cavara and Grande (Brassicaceae). We compared their morphology, oviposition behavior, larval development, distribution, abundance, and attack rates in their sympatric range, and of C. alliariae when found alone and in sympatry with C. roberti. Results indicate only very small differences in the fundamental niches of the two species. Comparison of C. alliariae in the range it occurs alone with the range where it co-occurs with C. roberti revealed some evidence for competition between the two species, i.e., attack levels of C. alliariae were reduced in areas where it co-occurred with C. roberti. However, the study showed no character displacement in regard to adult size or shoot choice of C. alliariae and we found no indication for superiority of either of the two species. Clearly, manipulative experiments would be necessary to unambiguously test for competition between the two species. Our results, based on a subset of niche dimensions known to be important in other systems, suggest that C. alliariae and C. roberti may present one of the rare cases, in which niche differentiation is not the main mechanism underlying coexistence.