Measurements of ammonium and nitrte uptakes bynatural phytoplankton assemblages from Jiaozhou Bay at various combinations of amencnium and nitrate concentations with 15N tracef techniques showed that uptake rate of cither nitrogen was influenced by the presence of the other but that the influence of ammonium on nitrate uptake was much greater that thatof nitrate om ammonmium uptake. The influence mechanism of ammonimum on iitrate uptake marifested as competition at lower concemtrations and as inhibition at higher concentrations (ammocium concentration>0.6umol/L). but no total inhibtion appeared within the concentration ragen of the eperiments (0-10 uml/L). The influence of nitrate on ammonoum uptzake seems to be a results of competiotion for uptake sites on the cell surface. In view of the in situ nutrient concentration in the given marine emironment, it is believed that both nitrogen souers are utilzed by phytoplankton. Nitrate uptake in the presence of ammorium and ammonoum uptake in the pressence of nitrate are exprssed by a non-lincurly and a linearly calibrated mMichaelis -Menten equation respectively. In addition, A non-linear fratio model is given withic may be useful in practicl studies on newproduction in Jiaozhou Bay.