The aquifer that supplies water to most channel catfish (Ictalurus punctatus) hatcheries in the Yazoo Basin of the Mississippi River flood plain in west-central Mississippi has a hardness of <10 mg/liter as CaCO3. Calcium hardness <10 mg/liter is known to reduce survival and growth of catfish sac and swim-up fry, but effects of low hardness on fingerling growth and survival have not been determined. We examined the effects of hardness (5 mg/liter and 67 mg/liter as CaCO3) on survival and growth of fingerling (2.0-3.2 g initial weight) channel catfish (USDA-103 line), blue catfish (I. furcatus, D&B line), and channel X blue catfish hybrids (USDA-103 channel X Rio Grande blue catfish) in 80-liter aquaria. Survival was ?99% in all treatments over the 33-day experimental period. Low calcium resulted in significantly (P < 0.05) reduced net weight gain in both channel (4.4±0.07 g vs. 3.6±0.08 g in high and low calcium treatments, respectively) and blue catfish (2.5±0.11 g vs. 1.9±0.11 g), but there was a significant treatment X species interaction, with hybrids growing faster (2.4±0.14 g vs. 2.8±0.09 g) in the low calcium water.