m ISO SWS spectra of the starburst galaxy NGC 3256.
We have observed many fine-structure lines of various atomic species as well as
three rotational transitions of molecular hydrogen. From the [SIII]
18 m/33 m line ratio we infer low electron densities of 
300
cm-3 in the HII regions. The observed [NeIII]/[NeII]
(15 m/12 m), [ArIII]/[ArII] (8.9 m/6.9 m), and
[SIV]/[SIII] (10.5 m/18.7 m) line ratios are consistent with an
ionizing radiation field with an effective temperature of 41000 K.
Comparison of the observed fine-structure line ratios with theoretical models of
nebular emission from evolving starbursts shows that stars with masses
50 M 
have been forming recently in NGC 3256. The H 
observations reveal the presence of warm molecular gas. We estimate that
109 M of H gas is present at temperatures close
to 150 K. This mass corresponds to a few percent of the total cold molecular
mass as estimated from CO studies.