Abstract: Synthetic lepidocrocite (gamma ferric oxide monohydrate) having very fine-grained, needle-like crystalline particles, with at least about 70% of the particles having a length to width ratio of greater than 10:1, and a length of up to about 2 microns is made by first oxidizing a vigorously agitated alkaline-treated ferrous chloride solution until the pH is about 2.9-4.1 to form a synthetic lepidocrocite collodial seed slurry. The oxidation of the colloidal seed slurry is continued (optionally in the presence of metallic iron) at a temperature about 80.degree.-140.degree. F. and pH about 2.9-4.1 in the presence of excess ferrous chloride, until from about 1.2 to 5 parts by weight of total product is formed per part by weight of seed. A synthetic magnetic gamma ferric oxide may be made from the synthetic lepidocrocite by optionally coating the lepidocrocite particles with an organic surfactant, reducing, oxidizing and mechanically densifying the gamma ferric oxide particles.