SWS SPECTROSCOPY OF THE STARBURST GALAXY NGC-3256

We present 2.5-40 mu m ISO SWS spectra of the starburst galaxy NGC 325 6. We have observed many fine-structure lines of various atomic specie s as well as three rotational transitions of molecular hydrogen. From the [SIII] 18 mu m/33 mu m line ratio we infer low electron densities of approximate to 300 cm(-3) in the HII regions. The observed [NeIII]/ [NeII] (15 mu m/12 mu m), [ArIII]/[ArII] (8.9 mu m/6.9 mu m), and [SIV ]/[SIII] (10.5 mu m/18.7 mu m) line ratios are consistent with an ioni zing radiation field with an effective temperature of NN 41000 K Compa rison of the observed fine-structure line ratios with theoretical mode ls of nebular emission from evolving starbursts shows that stars with masses greater than or equal to 50 M. have been forming recently in NG C 3256. The H-2 observations reveal the presence of warm molecular gas . We estimate that approximate to 10(9) M. of H-2 gas is present at te mperatures close to 150 K. This mass corresponds to a few percent of t he total cold molecular mass as estimated from CO studies.