In 1989 extensive biting by the black flies Simulium ochraceum was first reported on Isla San Cristóbal, Galapagos Is. It was assumed that this species was a recent human introduction. S. ochraceum is a vector for River Blindness on continental South America. Given the amount of human movement through the Galapagos from the continent, the arrival of the disease agent is a very real possibility. The discovery of black flies on the Galapagos represents a unique opportunity to study the evolution of the group. Although this family has colonized most other islands with running water in both the Atlantic and Pacific oceans, this is the first time a single, recent colonization event has been observed. The understandable determination of the Ecuadorean National Parks to eradicate this species makes the study of this population urgent. Should this species turn out to be endemic, then it is likely to be a keystone species, providing essential nutrients to other aquatic macroinvertebrates. Hence its eradication could irreversibly damage the ecological balance of streams on San Cristóbal. We present preliminary data to suggest that S. ochraceum has been a resident on San Cristóbal for a long time.