The fluxes, concentrations, and carbon isotopic compositions of soil CO2 were measured along the Parkfield segment of the San Andreas fault (SAF). Single or double-peak CO2 flux anomalies (>18 g m-2 d-1) were observed along 12 of 16 fault-crossing transects at five sites. Flux anomalies did not coincide with concentration anomalies. Flux anomalies paralleled the fault trace, suggesting zones of high diffusivity/permeability. One flux anomaly may have been accentuated by a 0.3 mm creep event. However, values of Δ13C (-23.7 to -21.6 ?) and 14C (110.5 to 111.9 pmC) for soil gas CO2 are characteristic of CO2 of biogenic origin. The CO2 flux anomalies are therefore consistent with fault-related biogenic gas flow and do not yield evidence for degassing of deeply derived CO2. ¿ 2000 American Geophysical Union