To carry out baseline studies on monitoring systems for red tides in Jinhae bay, measurements and analyses were made on seawater samples from 15 sampling stations during 15 months from July, 1979. Water quality parameters studied are temperature, pH, DO, salinity, COD, SS, NO$sub$3/, NO$sub$2/, PO$sub$4/, SiO$sub$2/, Ca, Mg, Cd, Cu, Pb, Zn, Chlorophyll ${alpha}$, diatoms and dinoflagellates. Multiple regression analyses were undertaken with chlorophyll ${alpha}$, cell numbers of diatoms and dinoflagellates as the dependent variables and water quality parameters as the independent variables. The results showed that biomass, expressed as total cell numbers of diatoms and dinoflagellates, was largely influenced by COD, salinity and nutrients.