Most hybrid catfish are produced by fertilizing eggs from hormone-induced, strippable channel catfish (Ictalurus punctatus) females with sperm from blue catfish (I. furcatus). Water to most hatcheries is supplied from 300 to 400-m deep aquifer, yielding geothermal water of 25-30 C with low level of calcium hardness and hence supplemented with an external source of calcium. Many catfish hatchery water sources have low calcium concentrations and are supplemented with an external source of calcium. Nevertheless, failure of calcium pump or delivery system in commercial catfish hatcheries is not uncommon. This study examined 12 sequential 8-hour periods of exposure of hybrid catfish eggs to calcium-deficient waters from fertilization to hatch. Periodic exposure to calcium-deficient waters did not affect fertilization of hybrid catfish eggs. However, any exposure period of hybrid catfish eggs to calcium-deficient waters (0.4 mg/L) within 48 h post-fertilization reduced hatching success of hybrid catfish eggs compared to a control group not exposed to calcium-deficient waters. This 48-hour post-fertilization period appeared to be a calcium-critical period in hybrid embryo development. Frequent monitoring of calcium hardness in hatchery waters is critical to minimize losses in hybrid catfish fry production, especially during the first 48 h post-fertilization.