The intake and the tissue concentration of Sulfamonomethoxine were studied with Japanese anchovy (Engraulis japonica) fed with a diet containing Sulfamonomethoxine sodium. The intake of Sulfamonomethoxine sodium was calculated at 257mg/kg body weight (128.5%) from the whole body concentration of tested anchovy when the drug was orally administrated at a rate of 200 mg/kg estimated total fish weight. This high value of intake rate over 100% seemed to be due to an overestimation of total fish weight. The intake ranged from 181 to 355 mg/kg body weight, individual variations being relatively little. The tissue concentration of Sulfamonomethoxine was 2.5 mg% (Free) and 3.1 mg% (Total) 6 hours after administration, and 0.2 mg% (Free) and 0.8 mg% (Total) hours after 24.