Two species of flea beetles, Aphthona lacertosa Rosenheim and A. nigriscutis Foudras (Coleoptera: Chrysomelidae), used to control leafy spurge, Euphorbia esula L., often co-occur when agents are collected for redistribution. There are two questions that arise when relocating insects to new areas, is successful establishment of A. lacertosa and A. nigriscutis dependent upon initial release size and is there interspecific competition between these biological control agents? Two experiments were carried out over three field seasons to determine if the number of Aphthona spp. initially introduced to a leafy spurge infestation would influence their establishment and whether interspecific competition between the two species influences establishment. Results of the release rate experiment indicate that both species were able to establish viable reproducing populations when release rates ranged from 250 to 1,000 and that A. lacertosa populations were significantly larger than what was observed for A. nigriscutis across all initial release rates. Results of the competition experiment do not indicate that interspecific competition was influencing establishment success of Aphthona spp. Two years after introduction, A. lacertosa populations either held steady or increased in all treatments, whereas A. nigriscutis populations tended to decline either in the presence or absence of A. lacertosa. We concluded that environmental conditions in Minnesota are more suitable for A. lacertosa establishment and population growth and that interspecific competition is not a major factor in establishing either species. We suspect that habitat or local climate has a major influence on Aphthona spp. establishment.