A study was conducted in a Louisiana coastal marsh to determine the effects of the insect growth regulator methoprene on aquatic organism populations. Six aerial applications of methoprene (28 gm AI/ha) during an 18-month period caused statistically significant differences in certain aquatic population, when treated and untreated populations were compared. Methoprene caused highly significant (P<0.01) reductions in natural populations of the scud. Hyalella azteca (Saussure) adults and young; opossum shrimp, Taphromysis louisianae (Banner) adults and young; freshwater prawns, Palaemonetes paladosus (Gibbs) adults and young; mayflies, Callibaetis sp. naiads; dance flies, Notophila sp. larvae; midges, Chironomidae larvae; fresh water snail, Physa sp. adults and young; damselflies and dragonflies, Enallagma, Anax and Belonia spp. naiads; burrowing water beetles, Suphisellus sp. adults and Hydrocanthus sp. adults; and water scavenger beetles, Berosus infuscatus Leconte adults and Berosus spp. larvae. Populations of the water boatmen, Trichocorixa louisianae Jaczewaski nymphs; moth flies, Psychoda sp. larvae; crawfish, Procambarus clarki (Girard) and Cambarellus sp. adults and young and predaceous diving beetle, Liodessus affinis (Say) adults significantly (P < 0.05 - 0.01) increased after the methoprene applications. No statistically significant (P > 0.05) difference was determined between the population numbers of 28 aquatic organisms when treated and untreated populations were compared. The data indicated that of those populations significantly reduced by the methoprene applications, 7 organisms were collected in significantly greater numbers from the habitat containing emergent vegetation, 2 organisms were more abundant in the open water habitat and 5 organisms were collected in almost equal numbers from both habitats. Those organisms not affected by the methoprene treatment were collected in greater numbers from the vegetative habitat (24) followed by the open water habitat (6) and 3 organisms showed no preference for either habitat.