Herbs glabrous or very sparsely strigillose distally. Stems erect, usually unbranched near base, branched at each of several nodes proximal to first flower, less branched distally, branch­ing dichotomous, 20–70 cm. Leaves much reduced distally, 10–60 × 1–4 mm; petiole 0–10 mm; blade very narrowly lanceolate to sublinear. Inflores­cences with flowers arising usually as proximally as first 10–20 nodes from base. Flowers: sepals 1–1.4 mm, reflexed singly or in pairs; petals 1.5–2.5 mm; pollen 90–100% fertile; stigma subglobose, surrounded by anthers at anthesis. Capsules ascending to strongly reflexed, subterete, 4–9 × 0.9–1 mm, with conspicuous constrictions between seeds, valve margins undulate, all valves free from septum after dehiscence, septum sinuous; pedicel 3–11 mm, usually about equal to capsule. Seeds usually 3–6, all developing, arranged ± parallel to septum and in alternating pattern between locules, adjacent seeds not overlapping, well spaced from each, forming a single row in capsule, brown, sometimes mottled with darker spots, 1.3–1.7 × 0.6–0.8 mm, glabrous. 2n = 14.

Flowering Jun–Sep. Open pine forests; 1300–2800 m; Calif.

Gayophytum oligospermum is known only from Los Angeles, Riverside, San Bernardino, and San Diego counties in southern California.

Gayophytum oligospermum is morphologically similar to G. heterozygum and has been shown to have chromosomes that pair completely with one of the two arrangements that make up the genome of the complex heterozygote G. heterozygum, indicating that G. oligospermum is one of the parents of G. heterozygum or that it is derived from G. heterozygum (L. B. Thien 1969).

Gayophytum oligospermum has been reported from the summit of Breckenridge Mountain, Kern County, from a single collection (Charlton 3750, UCR). This record needs to be verified.