The effects of sediment and water movement on the survival and growth of Campylaephora hypnaeoides J. Agardh were examined in culture to elucidate why this species grows as an epiphytic alga. The survival and growth rates of Campylaephora tetraspores declined as the sediment concentration increased under both water-movement and no-water-movement treatments. After 5-day cultures under various sediment treatments, the respective survival and growth rates ranged from 27.32 to 78.63% and 0.22 to 0.29 $day^{-1}$ under the no-water-movement and from 15.66 to 82.69% and 0.19 to 0.31 $day^{-1}$ under the water-movement treatments. The maximum survival and growth of C. hypnaeoides tetraspores occurred at 20 and 40 ${mu}mol$ photons $m^{-2}s^{-1}$, respectively. The survival of Campylaephora tetraspores and germlings declined with increasing dark period, but the tolerance of total darkness based on the survival rates was two-times greater for 5-day old sporelings than for tetraspores after 12 days. In conclusion, Campylaephora hypnaeoides grows as an epiphytic alga because its survivorship and growth decline rapidly with greater water movement and sedimentation and with lower light intensities.