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The BL Lacertae object, PKS2155-304, toward $l^{II}=17,b^{II}=-52$, has been observed both with the Extreme Ultraviolet Explorer (EUVE) and the Goddard High Resolution Spectrograph (GHRS) aboard the Hubble Space Telescope (HST). The EUVE observations show no detectable Extreme-UV flux longward of 100 \AA, but shortward of 100 \AA\ the decreased interstellar attenuation yields clearly detectable flux from the continuum of PKS2155-304. The EUVE detection togehter with the deduced H~I column density from the G140L data for H~I Ly$\alpha$ obtained with the HST/GHRS are used to obtain constraints upon the ionization of hydrogen and helium through the galactic halo toward PKS2155-304. Constraints for the ionization of H and He are presented based upon assumptions for the intrinsic EUV fluxes of the observed BL Lacertae object. More definitive constraints will be provided with further EUVE observations.
