Spatially growing disturbances in an alongshore current are considered. In many other types of shear flows these spatially growing modes describe the initial development of instabilities more satisfactorily than do the more commonly considered temporally growing modes. Here the spatial stability theory is applied to these wave-driven, semi-infinite types of flows for the first time. Both model <Thornton and Guza, 1986> and measured <Reniers et al., 1994> longshore current and beach profiles are analyzed, using both the temporal and spatial theories. Results of the two theories are compared using the approximate relations of Gaster <1962>. For the Thornton and Guza profile, predictions of the spatial theory and those from the transformed temporal theory are seen to be extremely close. For the profile of Reniers et al., which exhibited destabilization and spatially growing instabilities, the two sets of predictions are again very close. This implies that the simpler temporal stability theory, which has been used exclusively in previous studies, may be applied even when the spatial theory would clearly be more appropriate (such as in the experimental study examined here) by transforming the temporal predictions via Gaster's relations. Moreover, it appears that the predicted wavelength and period of the fastest growing mode (FGM) of untransformed temporal theory will be fairly good indicators of the same quantities for spatial modes and perhaps may be used as such when estimates of the group velocity from the temporal theory cannot be made with much certainty, such as when points are very sparse. It is shown that the basic structure of the fastest growing disturbance or mode (FGM) is similar for both theories for the Reniers et al. profile, although it would be a mistake to rely on the details of the linear eigenfunctions given by temporal theory. ¿ American Geophysical Union 1996