Although threadfin shad (Dorosoma petenense) have been widely introduced as forage fish, little is known about their natural history in tropical reservoirs. Gonadosomatic index (GSI) analysis of threadfin shad from Lucchetti Reservoir, Puerto Rico, indicated that most spawning occurred from January to June and again in October, but some spawning occurred year-round, except from mid-August to mid-September. Aging of sagittal otoliths indicated that total length (TL) increased with age (r2 = 0.36). Maximum length was 86 mm TL (N = 2,002) and maximum age was 141 days (N = 124). Hatch date distribution corroborated GSI data regarding spawning periodicity. Diet analysis indicated that detritus and insects were the primary foods. Inconsistencies in threadfin shad dynamics among reservoirs suggest that system-level approaches should be employed in predator-prey management when threadfin shad comprise an important part of the prey base.