The recovery planning process for the South-Central and Southern California steelhead Distinct Population Segments is currently underway, yet several important questions regarding population structure of coastal trout in southern California remain. To provide insight into questions of population structure in this geographic area, we have performed genetic analysis of samples from 7 basins in the two using microsatellite DNA, highly variable genetic markers that can be used to trace ancestry and evaluate even small genetic distinction among populations.  This study employ a collection of microsatellite loci to examine the genetic structure of O. mykiss in these two southernmost DPSs in California, with a focus on relationships between populations above and below dams. We analyze samples collected in a systematic effort in 2003 from 5 watersheds: the Salinas, Arroyo Grande, Santa Ynez, Ventura and Santa Clara Rivers. We also analyze samples collected opportunistically and in small numbers from the southernmost extent of the range. These include samples from Malibu and Topanga Creeks in Los Angeles County, the Santa Ana and Arroyo Trabuco basins in Orange County, and San Mateo Creek and the Sweetwater River in San Diego County. We also analyze samples of the O. mykiss strains raised at Fillmore Hatchery on the Santa Clara River and used in stocking of trout in reservoirs throughout the southern part of the state.