Blacklight traps at a density of 3/mile 2 were operated for 43 months (1966–69) on St. Croix, U. S. Virgin Islands, an isolated tropical island. Insects were collected regularly from ⅓ of the traps to obtain an index of the size of the population of 17 insect species. The species studied were: Acrosternum marginatum (Beauvais), Bothynus cuniculus (F.), Conoderus sp., Ecpantheria icasia (Cramer), Gryllus assimilis (F.), Heliothis virescens (F.), H. zea (Boddie), Jadera rufofusca Barber, Lacon subcostatus (Candeze), Loxa flavicollis (Drury), Microcentrum triangulatum Brunner, Neoconocephalus triops (L.), Nezara virdula L., Phyllophaga microphylla Moser, P. portoricensis (Smyth), and Schistocerca pallens (Thunberg). The populations of most species declined in 1967 and 1968 but increased in 1969. The decreases may have been related to the trapping and the increases to an abnormally high rainfall in 1969.