Plants narrow, medium-sized, epiphytic, sometimes terrestrial. Stems erect or pendulous, 2-to 12-forked, 13-28 x 0.2-0.4 cm, branches very slender, less than 2 mm wide. Sterile leaves dense, lying flat, imbricate, small, linear, very narrow, 2-4 x 0.1-0.15 mm, curving in at tips. Fertile leaves near branch tips, broadly ovate, margins entire, extending to long, pointed tips. Sporangia small, almost concealed by ovate bases of fertile leaves.

Found in wet forests, 335-1,920 m, all major islands except Kaua'i. On O'ahu, it is most frequently found epiphytic on Hibiscus arnottianus. Huperzia filiformis is also native to tropical America.

Huperzia filiformis may be distinguished by its thin, multibranched stems densely covered with narrow, pointed leaves overlapping and lying flat against the stems. These overlapping, needlelike leaves and lack of a clearly separated fertile tassel separate it from species in the Phlegmaria group.

Reference: 0llgaard (1987).