In a newly formuiated growth medium lacking Na+ and NH4I, Saccharomyces cerevisiae grew maximally at 5 ,IM K+. Cells grown under these conditions transported K+ with an apparent Km of 24 ,uM, whereas cells grown in customary high-K+ medium had a significantly higher Km (2 mM K+). The two types oftransport also differed in carbonyl cyanide-m-chlorophenyl hydrazone sensitivity, response to ATP depletion, and temperature dependence. The results can be accounted for either by two transport systems or by one system operating in two different ways.