The effects of temperature, light, and salinity on the growth of Costaria costata and Undaria pinnatifida juveniles were examined in laboratory cultures. In a cultivation farm, the monthly yield and density were also investigated between December and April for C. costata and between December and March in 2007 and 2008 for U. pinnatifida. The relative growth rates (RGRs) were greater at $20{sim}60;{mu}mol$ photons $m^{-2}s^{-1}$ than at low ($0{sim}10;{mu}mol$ photons $m^{-2}s^{-1}$) and high ($100{sim}180;{mu}mol$ photons $m^{-2}s^{-1}$) irradiance levels. The optimal growth conditions for the two species were $17^{circ}C$, 35 psu, $60;{mu}mol$ photons $m^{-2}s^{-1}$, and a daylength of 12 h, indicating that C. costata and U. pinnatifida have very similar growth responses to temperature, light, and salinity. However, the growth responses of the two species to various environmental factors were different; C. costata grew faster than U. pinnatifida but the latter species grew well at low salinity. The monthly yield of C. costata and U. pinnatifida increased steadily over the study period, and it was maximal in March for both species, but the yield of U. pinnatifida was greater than that of C. costata.