We present 2.5-40 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 2 observations reveal the presence of warm molecular gas. We estimate that 109 M of H2 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.