The black oaks of California include 4 tree species (California black oak, coast live oak, Shreve oak, interior live oak) that are known to hybridize. Complex patterns of population variation within each species are likely to result from these hybrid combinations and from subsequent introgressions. We have been studying population variation using biochemical and molecular markers and report results from the former here. Diversity is much greater in interior live oak and in Shreve oak than in either of the other two species, and is least in coast live oak. Shreve oak has not received complete acceptance as a valid species, and is considered as synonymous with interior live oak by many ecologists. However, our biochemical data provided a marker that was present in all populations identified as Shreve oak from the central coast, and was extremely rare in populations of interior live oak from the Cascade and Sierra Nevada mountains. This marker occurred at intermediate frequencies in many populations from north western California that were tentatively assigned to interior live oak. We suggest that these two species are recently derived from a common ancestor and that interspecific barriers to fertilization have not yet become complete. Discriminant function analysis on the full biochemical data set suggested a complex pattern of introgression including coast live oak, interior live oak and Shreve oak in these north coastal populations. These studies of population variation help us to understand the genetic architecture of the black oaks of California and may provide valuable information in the search for resistance to sudden oak death (SOD).