To assess the effects of gamma radiation at low doses, spores of Penicillium expansum NRRL 971 and Penicillium patulum NRRL 989 were exposed to doses of 0, 50, 100, 150, and 200 krads. The amount of patulin for each culture was quantitatively determined and growth at each dose was evaluated. The results showed a 31% increase in the amount of patulin synthesis after a dose of 150 krads for Penicillium expansum At 200 krads, both fungi showed low levels of patulin production and inhibited growth. Eight color mutants of P. expansum and eight color mutants of P. patulum were produced by exposing spores of each fungus to doses of 150 and 200 krads of gamma radiation. An evaluation of patulin production and growth for each mutant showed that color mutation could result in mutants with an enhanced ability to grow or produce increased levels of patulin. When grown at a subatmospheric pressure of 320 mm Hg, P. expansum produced 30% more patulin per flask, when compared to control cultures. At 160 mm Hg, P. expansum showed a 75% decrease over the control cultures. P. patulum was greatly affected by the temperature of 15℃ and showed decreases both in growth and patulin production. Effective control of growth and patulin synthesis was found for these fungi at 160 mm Hg.