The aim of this study was to evaluate the antimicrobial effect of two different copaiba oils against Escherichia coli obtained from mastitic milk. An antimicrobial assay was performed using an agar diffusion test, testing three solutions: a negative control and two solutions of different copaiba oils. This test was performed with 27 mastitic E. coli isolates. Eight isolates (29.63%) were inhibited by C. langsdorffii oil solution and seven by C. officinalis oil. The results of the present in vitro study suggest that copaiba oils may be potential sources of new and selective agents for the treatment of mastitis, pending further testing in in vivo models.