Abstract: When grown at pH4.5, Saccharomyces cerevisiae acquires a resistance to inhibitory acetic acid level (~0.1M) by destabilizing Fps1p, the plasma membrane aquaglyceroporin that provides the main route for passive diffusional entry of this acid to the cell. Acetic acid stress transiently activates Hog1p mitogen activated protein (MAP) kinase which, in turn, phosphorylates Fps1p in order to target this channel for endocytosis and a degradation in the vacuole. This activation of Hog1p is abolished with the loss of Fps1p, but more sustained when cells express an open Fps1p channel refractory to destabilization. At neutral pH much higher levels of acetate (~0.5M) are needed to inhibit growth. Under such conditions the loss of Fps1p does not abolish, but merely slows, the activation of Hog1p. Acetate stress also activates the Slt2(Mpk1)p cell integrity MAP kinase, possibly by causing inhibition of glucan synthase activity. In pH4.5 cultures this acetate activation of Slt2p is strongly enhanced by the loss of Fps1p and dependent upon the cell surface sensor Wsc1p. Lack of Fps1p therefore exerts opposing effects on the activations of Hog1p and Slt2p in yeast exposed to acetic acid stress.