Centers for Disease Control miniature light traps augmented with CO2 provide an effective method of monitoring Culex abundance and may provide a useful supplement to New Jersey light traps used by the California Mosquito Surveillance Program. To assist in standardizing sampling protocols, the present research compared the catch of adult mosquitoes collected using 4 trap designs and 3 CO2 presentation methods. When augmented with dry ice, the Arbovirus Field Station (AFS) trap (consisting of a 3-in. fan mounted into a white polyvinyl chloride pipe and operated without a light source or rain shield) collected as many or more Culex females than similar traps purchased from John W. Hock and American Biophysics, or a trap with a 4.25-in. 2-bladed fan constructed by the Orange County Vector Control District (similar to the Encephalitis Virus Surveillance model distributed by Bioquip). Few blooded or gravid females and males were collected, indicating that CO2 released from the dry ice and not light probably was the primary attractant. Catch of Culex tarsalis females in traps baited with CO2 released at 0.5-1.5 liters/min from gas cylinders was significantly greater than in traps baited with dry ice, even though the CO2 release rates from the dry ice at dusk probably were comparable to that released from the cylinders and averaged 0.4-0).5 liters/min for the night. Traps baited with 0.5 liters/ min of CO2 gas released in 15 3- or 2-sec bursts per hour collected the fewest mosquitoes. In all experiments, trap location effects were significant and accounted for as much variability in catch size as trap design or CO2 presentation. Sampling efficiency of all trap designs or CO2 presentations were consistent over time, space, and different levels of mosquito abundance.