Several molecular studies in the last 12 yr have made phylogenetic relationships among the nearly 50 species of Allium L. (Alliaceae) in California much better known than details of the plants' biology in the field. Allium jepsonii (Ownbey & Aase ex Traub) S. S. Denison & McNeal, a rare geophyte nearly endemic to Butte County, was studied in the field for 4 yr on a serpentine outcrop at 736 m elevation south of Paradise Lake, to document aspects of growth and reproduction. Plants flowered in late June and early July. Plant density, scape height and numbers of flowers (averaging 26) in the solitary umbels varied significantly through the years, with some variation in these traits related to cumulative rainfall. Each year many flower buds failed to reach anthesis and only 54% of the flowers that opened produced fruits. Bagged flowers produced significantly fewer fruits and seeds than unbagged. Common flower visitors each year included a beefly, Bombylius facialis. Seeds per plant varied by year, with the 4-yr average (200 fruits total) at only 14. Over the years 82% of the fruits made single seeds, with 17% making two and only 0.3% making three. Seeds planted in the field germinated in the winter, with seedlings completing first year growth by June. The cotyledon served as the only photosynthesizing leaf, and elongation of lower parts of the cotyledon pushed the developing bulb into the soil. First year bulbs averaged up to only 2.4 mm maximum diameter, were pearly white, and lacked the dark coats seen on bulbs of flowering individuals.