The fern Azolla filiculoides Lamarck (family Azollaceae) harbours a heterocystous nitrogen-fixing cyanobacterium Anabaena azollae Strasburger (Nostocales: Cyanobacteria) inside cavities on the dorsal lobes of the leaves. A new extra sheath was detected under scanning electron microscopy surrounding only the heterocysts of the cyanobiont and their chemical characterization by histochemical tests has pointed to the presence of proteins. The function of this extra sheath is unknown.