The Vegetarian Finch, Platyspiza crassirostris, is a relatively unstudied Darwin's finch that appears to be in decline in the Galapagos Islands. We monitored 11 nests of Vegetarian Finches during 2013 and 2014 on Santa Cruz Island, Galapagos, and found that 10 of these were infested with an invasive parasitic fly, Philornis downsi. This is the first report of P. downsi attacking this bird species. The number of P. downsi in nests of Vegetarian Finches was higher than for other Darwin's finch species, but nestling mortality was relatively low. We hypothesize that both of these trends may be related to fact that the Vegetarian Finch is one of the largest-bodied species of Darwin's finches. We also consider the conservation implications of P. downsi parasitism for populations of vegetarian finches and other Darwin's finch species in the Galapagos Islands.