The effects of Mg+2 and Ca+2 ion concentrations on Pichia stipitis growth and flocculation were studied by a 2x2 factorial design. Negative interaction was detected between these ions (p<0.01) : maximum growth rate was attained with high Mg+2 (3 mM) and low Ca+2 (0.34 mM) concentration levels, whereas the inverse ionic ratio was required to reach the maximum flocculation extent. Neither stimulatory or inhibitory effects on growth or flocculation were observed by increasing other cations (Zn+2, Mn+2, Co+2, Fe+3) or vitamins concentration. An antagonistic relationship between growth and flocculation was concluded.