Technical Abstract: Thirty species of the megachilid bee genus Osmia were recorded at a research reservation in central coastal California during two survey periods: 1937-43 and 1987-92. Diversity remained constant at 23 species between surveys. However, cumulative diversity increased from 23 to 30 species. The total number of species at this geographic locale is relatively high when compared with nine other surveys but is most typical of diversities found at other montane, mid-elevation latitudes. Differences between study periods suggest that long-term surveys are required to accurately assess species diversity.