A survey of the Boston Harbor Islands national park area yielded 175 lichen species in 67 genera, and 70 bryophyte species in 44 genera. Ten percent of the lichens represent a maritime floristic element, and 4% represent a coastal plain element. Caloplaca verruculifera, an orange lichen of rocky shores, is reported new for Massachusetts. Human disturbance and air pollution seem to be the major factors limiting the colonization of lichens and bryophytes on the islands, and specific recommendations for protecting sensitive lichen and bryophyte communities are presented.