Dr. Andrew Balfour, Director of the Wellcome Bureau of Scientific Research, has submitted to the writer for study part of the Diptera collected on his recent voyage in the American tropics. A species of Simulium from the valley of the Atrato River in Colombia proves to be new and is described below. Through the good offices of Dr. Balfour further interesting material was received from K. S. Wise, Surgeon-General for British Guiana, and from F. W. Urich, Entomologist of Trinidad. These lots contained an additional new species of Simulium each, of which the descriptions follow. Diptera belonging to other groups and of economic interest will be treated in subsequent papers.