Mimulus lewisii and M. cardinalis are closely related, but M. lewisii is pink-flowered and pollinated by bumblebees while M. cardinalis is red-flowered and pollinated by hummingbirds. Pollinator preference is believed to he due in part to the difference in flower color between these two species. Ten anthocyanins are characterized from M. lewisii and M. cardinalis petal lobes by high performance liquid chromatography (HPLC) and electrospray tandem mass spectrometry (ES/MSMS). Total anthocyanin concentration in the petals of M. cardinalis is approximately twice as high as that in M. lewisii, and that this difference can be accounted for by the presence of two pelargonidin biosides, probably rhamnohexosides, unique to M. cardinalis. Cyanidin biosides comprise 33\% of all the anthocyanins in M. lewisii, but only 2.4\% of those in M. cardinalis. (C) 1997 Elsevier Science Ltd.