The combined effect of three initial levels of oxygen (0, 10, and 20%), irradiation dose (0, 0.5, and 1 kGy), and storage temperature (5, 15, and 25°C) on toxin production by Clostridium botulinum in inoculated modified atmosphere packaged pork were investigated using factorial design experiments. Toxin was detected after only 2 d in all treatments stored at 25°C. At 15°C, irradiated and nonirradiated product packaged with 10 or 20% headspace oxygen were toxic after 14 d. For product packaged with 0% oxygen and an oxygen absorbent, toxin was detected after 21 d in nonirradiated samples compared to 43 d for product treated with an irradiation dose of 1 kGy. No toxin was detected in any product stored at 5°C, even after 44 d. Headspace oxygen in product initially packaged with 20% oxygen decreased to 0.1% after 14 d at 15°C and to ≤3% after 5 d at 25°C, with a concomitant increase in package headspace CO2 to 25–40%. For product packaged with 0% O2 and an oxygen absorbent, oxygen remained at ≤2% throughout the storage trial, while CO2 increased to 10 and 24% for nonirradiated and irradiated samples, respectively. Initial packaging of product with O2 appeared to enhance toxin production by C. botulinum in product stored at 15°C, probably as a result of increased CO2 enhancing spore germination.