Fumigation with either gas cartridges or aluminum phosphide is often considered to be an effective method for controlling California ground squirrel populations, although few studies have corroborated their effectiveness. Additionally, utilizing aluminum phosphide in combination with gas cartridges could increase effectiveness of both fumigants by indicating additional burrow openings through escaping smoke from gas cartridges and allowing for treatment of these additional openings with aluminum phosphide, which is a cheaper and potentially more effective material. Therefore, we compared the efficacy of gas cartridges, aluminum phosphide, and a combined treatment on ground squirrel populations from mid-April through early May 2009 to determine which was most effective. We established 4 plots (3 treatments plus control) at each of 2 treatment sites in Madera County, California. All plots were between 1 and 2 ha in size with survey areas of 0.3 - 0.5 ha centrally located within the plot. Ground squirrels were visually counted within survey areas for 3 consecutive days prior to treatment. Following counting, treatments were applied. Forty-eight hours post-treatment, squirrels were again counted within survey areas to determine percent control. These counts were repeated the following 2 days. All treatments were then reapplied for the same plots, with similar surveys again following 48 hours post-treatment. We found that all fumigation methods resulted in marginal to exceptional control of ground squirrels (percent control: gas cartridges = 62 - 86%, aluminum phosphide = 97 - 100%, gas cartridges + aluminum phosphide = 59 - 71%). We suggest aluminum phosphide use could be expanded to increase ground squirrel control in California, given its high efficacy combined with its relatively cheap material cost.