Activity coefficients for NaCl in aqueous solutions containing a variable molality of NH2CH2CO2H (glycine) were obtained from the e.m.f.s of the galvanic cell : Na-glass NH2CH2CO2H (m(A)), NaCl (m(B)), AgClAg, at different ionic strengths. They were fitted to a modified Pitzer equation by taking into account that the ion-zwitterion interactions depend on both the ionic strength and the zwitterion molality of the solution. Standard deviations for the fittings of about the same size as the accepted experimental uncertainty (+/-0.10 mV, i.e. +/-0.0008 in lg gamma+/-) were found by neglecting the ion-zwitterion-zwitterion and ion-ion-zwitterion interactions. Activity coefficients for NH2CH2CO2H in these aqueous mixtures were thus calculated. They agree well with the literature values obtained for more dilute solutions.