International trade frequently moves mollusks around the globe, thereby increasing their opportunity to cause ecological and economic damage. Recent genetic studies have confirmed the identity of South American applesnails (Pomacea insularum) in the southeastern US, but limited literature exists on this species. Understanding fecundity provides direct insight into the invasive potential of mollusks. Our study documents P. insularum fecundity in Texas and offers comparisons with the closely related global invader P. canaliculata. We quantified P. insularum clutch and hatchling physical characteristics and examined field and laboratory hatching success. Clutches contained thousands of eggs (mean = 2064 eggs), and clutch size tended to increase over the reproductive season. Clutches exhibited average field and laboratory hatching efficiencies (number of hatchlings/total number of eggs) near 70 and 30%, respectively. Notably, several clutches hatched at 80% or higher in the field. Exotic P. insularum deposit more eggs/clutch than the related invader P. canaliculata, but we do not yet know how seasonal hatching efficiencies compare. However, even with a conservative estimate of 1 to 10% survival to adulthood, the average P. insularum clutch yields 14 to 144 new applesnails. The high fecundity of P. insularum translates into considerable ecological impact because adult females can contribute >1 clutch/wk over an extended growing season in the southeastern US. The need for research has increased with the emergence of P. insularum populations in the fragile Florida Everglades. We advocate life-history studies like ours to help understand the invasion potential of applesnails and other invasive mollusks.