The California ground squirrel (Spermophilus beecheyi) is widely distributed throughout California. It causes serious damage to agricultural crops. Tests were conducted to evaluate the fumigant aluminum phosphide (Phostoxin®) and to compare it to the commonly used gas cartridge. Treatments consisted of applying either a single-dose (two 3-gm tablets) or double-dose (four 3-gm tablets) of aluminum phosphide, or 1 U.S. Fish and Wildlife Service gas cartridge in each burrow in the study plots. The burrow opening was then plugged with 1 sheet of newspaper and sealed with soil. Burrows in the control plot were plugged with newspaper and sealed in the same manner. All plots were retreated 4 days after the initial treatment. Overall, the aluminum phosphide treatments were more effective than the gas cartridge treatments in terms of the reduction in the number of ground squirrels seen and in reopened burrows observed after treatment. The single-dose aluminum phosphide treatment was as effective as the double-dose treatment. The gas cartridge treatment required a greater amount of time for application and did not achieve very effective control. Many aspects of this fumigation technique remain unknown and continued research is necessary.