SOME termites can live on a diet of cellulose filter paper1. This characteristic prompted Hungate2 to question the possibility of N2 fixation in termites by estimating the N balance of growing termite colonies, but he found no net increase in total colony N. Toth3, on the other hand, incubated macerated termites in an N-free organic medium and observed an increase in fixed N in the solution. He used long incubation times, however, and possibly selected for N2-fixing organisms in the medium. We decided to reinvestigate the question of N2 fixation in termites by exploiting the sensitivity of the acetylene reduction assay, a reliable indicator of N2 fixation4,5. Preliminary reports of these results have already been published6,7.