Ecological data are lacking for many species covered in multi-species habitat conservation plans and other regional conservation plans. Erodium macrophyllum (Geraniaceae) is a rare native annual forb found throughout California, southern Oregon, and northern Baja California (Mexico) for which very little ecology is known. I report on the habitat characteristics and current and historical distribution of E. macrophyllum by collecting data on populations in Riverside and San Louis Obispo Counties and by conducting an herbaria search and electronic-mail survey. Erodium macrophyllum population sizes range from a few individuals to over 1000 and are restricted to heavy clay soils. Most populations are found on the eastern side of the coast ranges in California. The clay soils on which it is found typically have low cover of native and exotic species but often have other rare species. They are also generally low in nitrogen and phosphorus, but there is considerable variation. Based on herbaria records and the e-mail survey, E. macrophyllum was probably once more common and may have occurred on other soil types.