We present the 2.4 - 45 \mu m ISO-SWS spectrum of the Be star \gamma Cas ( B0.5 IVe ) .
The spectrum is characterised by a thermal continuum which can be well fit by a power-law S _ { \nu } \propto \nu ^ { 0.99 } over the entire SWS wavelength range .
For an isothermal disc of ionized gas with constant opening angle , this correponds to a density gradient \rho ( r ) \propto r ^ { -2.8 } .
We report the detection of the Humphreys ( 6- \infty ) bound-free jump in emission at 3.4 \mu m. The size of the jump is sensitive to the electron temperature of the gas in the disc , and we find T \approx 9 000 K , i.e .
much lower than the stellar effective temperature ( 25 000-30 000 K ) .
The spectrum is dominated by numerous emission lines , mostly from H i , but also some He i lines are detected .
Several spectral features can not be identified .
The line strengths of the H i emission lines do not follow case B recombination line theory .
The line strengths and widths suggest that many lines are optically thick and come from an inner , high density region with radius 3-5 R _ { * } and temperature above that of the bulk of the disc material .
Only the \alpha , \beta and \gamma transitions of the series lines contain a contribution from the outer regions .
The level populations deviate significantly from LTE and are highly influenced by the optically thick , local ( disc ) continuum radiation field .
The inner disc may be rotating more rapidly than the stellar photosphere .