Simazine is commonly used to control broadleaf weeds and annual grasses in perennial tree and vine crops because of its relatively low cost and long residual activity. Simazine may be subject to enhanced biodegradation in some areas which can result in decreased herbicide persistence and reduced residual weed control. Laboratory studies were conducted to determine if rapid simazine degradation occurs in California citrus orchards and if degradation rates are correlated with simazine use history. In the Central Valley, simazine degradation curves indicate that simazine degradation rate is more rapid in soils with a simazine use history (adapted) compared to soils with no recent use (non-adapted). In these soils, simazine dissipation was two- to three-fold faster in adapted compared with the non-adapted soils. However, in southern California, simazine dissipation and mineralization were not substantially different among soils with different simazine use histories. Repeated simazine use in California orchards can lead to the development of enhanced microbial degradation of the herbicide. However, soil type and long-term cropping factors can affect persistence and distribution of herbicide-degrading microbial populations in California orchards.