Populations ofManduca sexta(L.) were reduced by 86% on St. Croix, U. S. Virgin Islands, when blacklight traps at a density of 3/mile2were operated for 43 months; during the last 22 months, most traps were baited with virgin female tobacco hornworm moths. The collections indicated that hornworms were present throughout the year and that 2 periods of high population occurred each year, both associated with rainy seasons. When the collections of females from 9 traps operated for a year before the mass trapping began were used as a base for comparison, the yearly reductions in the mean collections for 12 months ending in December were: 60% from 1966 to 1967; 51% from 1967 to 1968; and 24% from 1968 to 1969. Annual operation of blacklight traps during the periods of maximum activity of the tobacco hornworm moth would reduce populations of the insect in similar situations and probably also in continental land masses.