We studied nitrate-nitrogen uptake by sterile Ulva sp. under various conditions including tropical ones. The Sterile Ulva sp., which was collected from Yokohama, Japan, removed nitrate-nitrogen in the culture medium. The algae took in nitrate-nitrogen even without light. The uptake rate of nitrate-nitrogen decreased with the increase of the nitrate-nitrogen content in algal cells. The ambient ammonia-nitrogen lowered the rate of nitrate-nitrogen uptake, whereas the ambient nitrate-nitrogen did not affect the ammonia-nitrogen uptake.