  Spectroscopic measurements of emission line ratios and velocities are
presented for ionized gas across the central plane of NGC 55, a late-type
galaxy in the Sculptor group. The low metallicity in NGC 55 leads to relatively
low line ratios of [S II]/H$\alpha$ and [N II]/H$\alpha$ for H II regions as
well as the diffuse ionized gas. These are the first spectroscopical
measurements of line ratios in ionized gas exterior to the stellar disc of NGC
55. Analysis of the line ratios and the relative velocities of different
features suggests that photoionization is a plausible explanation for the
ionization of this diffuse gas. The observed shell structures and the
corresponding velocities support the idea of diffuse gas being pushed into the
halo by supernova explosions and stellar winds.
