Larvae of the pyralid moth,Sameodes albiguttalis(Warren), caused heavy but, sporadic damage to waterhyacinth,Eichhornia crassipes(Mart.) Solms in Argentina. Females laid an avg of 300 eggs each, usually in injuries, on the plant leaves. The larvae fed inside the petioles and bud of the plant and pupated in white cocoons in the bulbous-type petioles. Eggs required 4 days to hatch, the 5 larval instars took a total of 21 days to pupate, and pupae took 7 days to emerge. In the laboratory, 95.6% of the eggs survived, 54.3% of the larvae, and 96% of the pupae. With optimum conditions in the laboratory, the population was calculated to increase 1.16 times/day, or 150 times/generation of 34 days, and to double each 4.7 days. Five generations/year occurred in the field, and maximum populations measured were 130 larvae and pupae/l00 plants. A braconid parasitoid heavily attacked larval populations and a pathogen,Nosema invadensor nr., infected all stages in the field in Argentina.Sameodes albiguttaliswould probably be an effective control agent if released in regions where it is not native. A synergistic effect between it and weevils of the genusNeochetinais predicted. The factors affecting its potential effectiveness are discussed.