The flea beetle faunae associated with three cruciferous plants, Dentaria diphylla Michx., Barbarea vulgaris R.Br. and Brassica oleracea L. were investigated in the field near Ithaca, N.Y. The flea beetles were sampled regularly from these plants both in their natural habitats and in cultivated plots. The samples were taken either by visually counting the insects or with a D'Vac vacuum net. The behavior and microhabitat preferences of the beetles were determined by making quantitative observations and counting the individuals on vertical layers of the hosts and on host plants growing in different habitats. The flea beetle loads were calculated on the basis of dry weights of plants and flea beetles. Six flea beetle species were found on these three crucifers. On each plant species there was a dominant flea beetle species which constituted over 60 % of the total specimens collected. When the plants were grown in identical cultivated plots in an open field, the relative abundances of flea beetle species attacking them were greatly altered from the situation found in the natural habitats of the hosts. These results, together with the data on the seasonal abundances, behavior and microhabitat selection of the flea beetles, showed that the phenology and habitats of the plants greatly influence their utilization by different flea beetle species and partially determine the distinctiveness of the crucifer-beetle associations.